A couple patches to avoid unnecessary parent actor relayouts (and full redraws) on some usual operations from gnome-shell.

The main target of these changes is main.layoutManager.uiGroup, which is 1) prone to have a bunch of child actors, and 2) Is as big as the stage, so this is a source of sudden fullscreen redraws.

This is independent, but better used with gnome-shell!197 (merged)