windowManager: add animations for fullscreen and unfullscreen

windowManager: add animations for fullscreen and unfullscreen

We use the newly introduced feature from Mutter to hook up our own
fullscreen and unfullscreen animations.
To give the illusion of a transition as smooth as possible, we create a
snapshot of the current contents of the actor before its state is
changed, and crossfade between the two states while the size changes.

https://bugzilla.gnome.org/show_bug.cgi?id=707248