Decouple window effects from freezing/thawing of window actors

The size-change animation in gnome-shell needs to sync the window actors geometry during the animation, it currently does this by notifying the compositor that the animation was finished before it actually is.

This causes a few bugs in Mutter though, since it will now emit the "effects-completed" signal on the window actor even though they aren't completed.

To fix that, we need to decouple freezing and thawing of actor updates from window effects and allow gnome-shell to thaw actor updates before it notifies Mutter that the effect is completed.

Shell MR: gnome-shell!1251 (merged)