workspace: Don't unfreeze layout during overview animation

During overview closing animation the allocation expands more or less every frame. Unfreezing the layout then leads to windows snapping to their new position if e.g. a window was closed which doesn't look nice.

Fixes #3216 (closed)

I have a feeling with this particular change when triggering the above issue the windows slightly miss their final positions during the overview closing animation. Well, that still looks better than before. This is present on master as well, not a problem with my patch.

cc @verdre