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/mission_item.xml

@@ -1,5 +1,6 @@
 <RelativeLayout
 	xmlns:android="http://schemas.android.com/apk/res/android"
+	xmlns:app="http://schemas.android.com/apk/res-auto"
 	android:layout_height="wrap_content"
 	android:layout_width="match_parent">
 	
@@ -37,7 +38,7 @@
 				android:layout_alignParentRight="true"
 				android:layout_centerVertical="true"
 				android:layout_marginRight="1dp"
-				android:src="@drawable/ic_menu_more"
+				app:srcCompat="@drawable/ic_more_vert_white_24dp"
 				android:scaleType="center"
                 android:contentDescription="TODO" />