ContentView Refactor

* ContentView extends CompositeView.
* ContentView is composed of subviews (e.g. ContentHeaderView, TiledAttachmentListView, BlockAttachmentListView, ContentBodyView, ContentFooterView)
* Decouple content view structure from styling.
* Remove GalleryOnFocusView
This mixin should technically work on a contentView, but it looks like it's only tested and used with listViews.

Code doesn't necessarily need to change, but if we only test it against listViews, this free variable should probably be named that. And any @param jsdoc may want to specify it.

Alternatively, add a test to ensure that this could keep working if mixed into a contentView too. That may be nice for standalone contentVIews.