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/menu/menu_channel.xml

@@ -1,7 +1,7 @@
 <menu xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
-    tools:context="org.schabi.newpipe.ChannelActivity">
+    tools:context="org.schabi.newpipe.fragments.channel.ChannelFragment">
 
     <item android:id="@+id/menu_item_openInBrowser"
         app:showAsAction="never"
@@ -11,10 +11,4 @@
         android:title="@string/share"
         app:showAsAction="ifRoom"
         android:icon="?attr/share"/>
-
-    <item
-        android:id="@+id/action_settings"
-        android:orderInCategory="100"
-        android:title="@string/action_settings"
-        app:showAsAction="never" />
 </menu>