dash: Disable animations during overview transitions

When updating the dash, we already avoid all animations while the
overview is hidden. However, as we are using Main.queueDeferredWork(),
updates may be deferred up to ~20 seconds while the overview is hidden.
If the overview is entered before a queued update has taken place, it
will be run immediately on map - as the overview is visible by then,
this means animating any outstanding changes.
Work around this by skipping animations during overview transitions as
well.

https://bugzilla.gnome.org/show_bug.cgi?id=686530