Fix handling of SystemBackground

Since the background rework, SystemBackground is no longer a transparent
actor that you have to stack on top of a solid background, it is an
opaque actor. Fix the color of the background actor, and remove places
where we were setting the background color underneath the system background
and expecting blending - in particular, we can always set no_clear_hint
on the stage.

https://bugzilla.gnome.org/show_bug.cgi?id=738652