ListFragment: Replace GridView with RecyclerView.

With some nasty code in our RecyclerView.Adapter to handle the
Cursor, because there is no equivalent for CursorAdapter as with
ListView/GridView.
I suspect that all the Cursor.setPosition() calls are very bad for
performance.
ListFragment: Make the grid width/height match the parent.

It was not a problem, but it feels correct.
RecyclerView: imageView: Set android:background="?android:attr/selectableItemBackground"

This should apparently give use the "ripple" effect on click, but it
doesn't seem to work.
ListCursorAdapter (RecyclerView): Respond to cursor changes.

This code doesn't actually seem to work, but new items showed up
already anyway. Mysterious.
ListFragment: Try to make the ImageView have the ripple effect.

This doesn't actually seem to work, however.