window-actor: Destroy the actor despite stuck animations
Under very rare circumstances clients crash mid-animation at unexpected times. In the worst case this can lead to a window-actor not getting destroyed, stuck on the screen forever.
Since animations go through Gnome-Shell and therefore rely on relatively complex logic, make our destruction code a bit more robust, allowing it to destroy the actor even if unfinished animations are pending.
Fixes #863 (closed) and fixes #655 (closed)