global: Filter out destroyed actors from get_window_actors()

global: Filter out destroyed actors from get_window_actors()

All current code assumes that the list of window actors corresponds to the
list of windows; however as the list returned by meta_get_window_actors()
now includes actors during the destroy animation, that assumption breaks.

Eventually we should make everyone move to a more appropriate API, but
for now make it work again by returning a filtered list of "good"
window actors.

https://bugzilla.gnome.org/show_bug.cgi?id=735927