Workspace/WorkspaceThumbnail: fix Clutter warnings from stacking code

Windows can be restacked at any time, including when the stackAbove
property of the window clone is being dragged, and thus parented to
the uiGroup. To do stacking properly, we need to skip it for the duration
of the drag, and sync it again at the end (which is already done by
mutter because of the workspace change)

https://bugzilla.gnome.org/show_bug.cgi?id=685285