Since tweener can't do multiple tweens on the same property, it
calls an overwrite callback for any existing tweens when a new
tween is added for the same property. Here we use the overwrite
callback to tell mutter that the effects are done.
Previously we were only doing this check for switch-workspace, but it
makes sense to do it for all transitions, since they won't be visible
anyway, and they might interfere with the overlay's operations by moving
windows around unexpectedly.
svn path=/trunk/; revision=177
switchWorkspaceDone was being called with the wrong 'this', causing
ShellWM.completed_switch_workspace() never to be called, causing stacking
order to get confused.
http://bugzilla.gnome.org/show_bug.cgi?id=567091
svn path=/trunk/; revision=142
