Adapt to Mutter background changes

The rewrite of Mutter's background code (see bug 735637) requires
corresponding changes here - we no longer need to layer multiple
MetaBackgroundActors together.

The general strategy is that a BackgroundSource object is created
per GSettings schema, and keeps either one Background/MetaBackground pair,
or, for animation, a Background/Metabackground pair for each monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=735638
Test stuff