Implement new feed and subscriptions groups

- Introduce Groupie for easier lists implementations
- Use some of the new components of the Android Architecture libraries
- Add a bunch of icons for groups, using vectors, which still is
compatible with older APIs through the compatibility layer

app/src/main/java/org/schabi/newpipe/MainActivity.java

@@ -240,7 +240,7 @@ public class MainActivity extends AppCompatActivity {
                 NavigationHelper.openSubscriptionFragment(getSupportFragmentManager());
                 break;
             case ITEM_ID_FEED:
-                NavigationHelper.openWhatsNewFragment(getSupportFragmentManager());
+                NavigationHelper.openFeedFragment(getSupportFragmentManager());
                 break;
             case ITEM_ID_BOOKMARKS:
                 NavigationHelper.openBookmarksFragment(getSupportFragmentManager());