When the user is doing a drag-and-drop, we want to temporarily show the
stage to allow them to drag to a different window. But we're not "really"
in the overview, and getting a grab would conflict with the X client doing
the drag and drop.
So add a showTemporarily()/hideTemporarily() pair of methods that show
the overview without grabbing.
This adds a lot more possibilities for asynchronous race conditions, so
rework the code to be more robust against multiple calls to show*()
and hide*(). The interpretation is now that all calls to show*() and
hide*() affect the state, but if we have conflicting calls to show and
hide we wait until the current animation is finished before correcting
to the right visual state.
https://bugzilla.gnome.org/show_bug.cgi?id=601731
62507c9759