Use vector drawables instead of PNGs for material icons

For all manually-created images PNG have been kept.
- rename all icon attrs to have a `ic_` prefix
- always use `_24dp` icons, because there is no real difference, since they are vector drawables
- always use the original name found on material.io for icon drawables, as to not create confusion and possibly duplicates. Icon names can still be different from real drawable names, though I have made some of them compliant to this or maybe more meaningul.
- remove duplicate `getIconByAttr()` in ThemeHelper (use `resolveResourceIdFromAttr()`
- use standard icons for `expand_more` and `expand_less` instead of triangles
- use `play_button_outline` instead of custom PNG as play button in VideoDetailFragment (questionable, as there is no shadow anymore)

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

@@ -41,7 +41,7 @@
                 android:layout_width="@dimen/drawer_header_newpipe_icon_size"
                 android:layout_height="@dimen/drawer_header_newpipe_icon_size"
                 android:layout_marginEnd="@dimen/drawer_header_newpipe_icon_title_space"
-                app:srcCompat="@drawable/splash_forground"
+                app:srcCompat="@drawable/splash_foreground"
                 tools:ignore="ContentDescription" />
 
             <TextView
@@ -89,7 +89,7 @@
                     android:scaleType="fitCenter"
                     app:tint="@color/drawer_header_font_color"
                     tools:ignore="ContentDescription"
-                    tools:srcCompat="@drawable/youtube" />
+                    tools:srcCompat="@drawable/place_holder_youtube" />
 
                 <TextView
                     android:id="@+id/drawer_header_service_view"
@@ -107,7 +107,7 @@
                 android:layout_width="24dp"
                 android:layout_height="24dp"
                 android:layout_marginStart="4dp"
-                app:srcCompat="@drawable/ic_arrow_drop_down_white_24dp"
+                app:srcCompat="@drawable/ic_expand_more_white_24dp"
                 tools:ignore="ContentDescription" />
         </LinearLayout>
     </LinearLayout>