Previously, we initialized actor stacking order from the return
value of global.get_windows() once, which is defined to be in
stack order. However it was not updated later. Furthermore,
the way stackAbove was called from onAnimationComplete in
WindowClone was highly dubious, since there are lots of animations
which apply to the clones, and we want the stacking to be right
all of the time, not when some animation completes.
Fix this by connecting to 'restacked' on the screen and syncing
the clones.
I also snuck in another bugfix here; we weren't disconnecting
from the 'showing' signal handler, which had various bad
consequences.
https://bugzilla.gnome.org/show_bug.cgi?id=596263
20b29ff48f