Migrate to fragments and improvements

- Migrate to fragments
- Fix #487
- Don't show "Open in popup mode" to channel links
- New backstack of videos
- Change the subscribers count to format using `NumberFormat`, for example some locales use `.`  others `,`, this handles it automatically (and the old method had a bug for leading zero, e.g. 4.82.125 instead of 4.082.125)
- Add string 'subscribers' for channels with more than 1 subscriber (plural)
- Popup player chooses the default format and resolution based on the new preference (format)
- Fix taskaffinity of the router activities
- Show title before loading, as it is available from the items already loaded

app/src/main/res/layout/activity_main.xml

@@ -1,15 +1,16 @@
 <?xml version="1.0" encoding="utf-8"?>
-<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+<RelativeLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    tools:context="org.schabi.newpipe.MainActivity"
-    android:orientation="vertical">
+    android:orientation="vertical"
+    tools:context="org.schabi.newpipe.MainActivity">
 
-    <fragment
-        android:id="@+id/search_fragment"
+    <FrameLayout
+        android:id="@+id/fragment_holder"
         android:name="org.schabi.newpipe.search_fragment.SearchInfoItemFragment"
         android:layout_width="match_parent"
-        android:layout_height="match_parent" />
+        android:layout_height="match_parent"/>
 
 </RelativeLayout>

app/src/main/res/layout/fragment_channel.xml

@@ -3,24 +3,22 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
+    xmlns:tools="http://schemas.android.com/tools"
     android:orientation="vertical"
-    android:title="Channel">
+    android:title="@string/channel">
 
     <android.support.v7.widget.RecyclerView
         android:id="@+id/channel_streams_view"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:background="?android:windowBackground"
-        android:scrollbars="vertical"/>
+        android:scrollbars="vertical"
+        tools:listitem="@layout/stream_item"/>
 
-    <RelativeLayout
-        android:layout_width="match_parent"
-        android:layout_height="match_parent"
-        android:id="@+id/channel_loading">
-        <ProgressBar android:id="@+id/progressBar"
-            android:layout_width="wrap_content"
-            android:layout_height="wrap_content"
-            android:layout_centerInParent="true"
-            android:indeterminate="true"/>
-    </RelativeLayout>
+    <ProgressBar
+        android:id="@+id/loading_progress_bar"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_centerInParent="true"
+        android:indeterminate="true"/>
 </RelativeLayout>

app/src/main/res/layout/fragment_search.xml

@@ -5,7 +5,7 @@
     android:layout_height="match_parent"
     android:layout_width="match_parent"
     android:name="org.schabi.newpipe.SearchInfoItemFragment"
-    tools:context=".search_fragment.SearchInfoItemFragment">
+    tools:context=".fragments.search.SearchFragment">
 
     <include layout="@layout/main_bg" />
 
@@ -17,10 +17,11 @@
         tools:listitem="@layout/stream_item"
         android:scrollbars="vertical"/>
 
-    <ProgressBar android:id="@+id/progressBar"
+    <ProgressBar android:id="@+id/loading_progress_bar"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerInParent="true"
         android:indeterminate="true"
-        android:visibility="gone"/>
+        android:visibility="gone"
+        tools:visibility="visible"/>
 </RelativeLayout>

app/src/main/res/layout/fragment_video_detail.xml

@@ -17,6 +17,7 @@
             android:layout_width="match_parent"
             android:layout_height="wrap_content">
 
+            <!-- THUMBNAIL -->
             <RelativeLayout
                 android:id="@+id/detail_thumbnail_root_layout"
                 android:layout_width="match_parent"
@@ -57,14 +58,13 @@
 
             </RelativeLayout>
 
+            <!-- TITLE -->
             <RelativeLayout
-                android:id="@+id/detail_content_root_layout"
+                android:id="@+id/detail_title_background"
                 android:layout_width="match_parent"
-                android:layout_height="match_parent"
-                android:layout_below="@id/detail_thumbnail_root_layout"
-                android:background="?android:windowBackground"
-                android:visibility="gone"
-                tools:visibility="visible">
+                android:layout_height="wrap_content"
+                android:layout_below="@+id/detail_thumbnail_root_layout"
+                android:background="?android:windowBackground">
 
                 <RelativeLayout
                     android:id="@+id/detail_title_root_layout"
@@ -106,13 +106,25 @@
 
                 </RelativeLayout>
 
+            </RelativeLayout>
+
+            <!-- CONTENT -->
+            <RelativeLayout
+                android:id="@+id/detail_content_root_layout"
+                android:layout_width="match_parent"
+                android:layout_height="match_parent"
+                android:layout_below="@+id/detail_title_background"
+                android:background="?android:windowBackground"
+                android:visibility="gone"
+                tools:visibility="visible">
+
+
                 <TextView
                     android:id="@+id/detail_view_count_view"
                     android:layout_width="match_parent"
                     android:layout_height="wrap_content"
                     android:layout_alignParentLeft="true"
                     android:layout_alignParentStart="true"
-                    android:layout_below="@id/detail_title_root_layout"
                     android:layout_marginLeft="12dp"
                     android:layout_marginRight="12dp"
                     android:layout_marginTop="5dp"
@@ -309,14 +321,15 @@
 
             </RelativeLayout>
 
+            <!-- LOADING BAR -->
             <ProgressBar
                 android:id="@+id/detail_loading_progress_bar"
                 style="@style/Widget.AppCompat.ProgressBar"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
-                android:layout_below="@+id/detail_thumbnail_root_layout"
+                android:layout_below="@+id/detail_title_background"
                 android:layout_centerHorizontal="true"
-                android:layout_marginTop="30dp"
+                android:layout_marginTop="20dp"
                 android:indeterminate="true"/>
 
         </RelativeLayout>

app/src/main/res/layout/main_bg.xml

@@ -4,7 +4,7 @@
     android:orientation="vertical" android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:id="@+id/mainBG"
-    tools:context=".detail.VideoItemDetailActivity">
+    tools:context=".fragments.detail.VideoDetailFragment">
 
     <TextView
         android:layout_width="wrap_content"