Avoid missing/broken app launch animations

If an app launches and maps a window quickly then the overview is still visible (but closing). This means the user sees the overview window clone animations, which are different to the regular _mapWindow animation, briefly while the overview closes. And since the overview is technically still visible, the ideal _mapWindow animation is skipped, resulting in the final window appearing abruptly.

This proposal fixes all that. In the case where the overview is visible but closing we now disable the overview animations for new windows and defer to the proper window mapping animation after the overview fully closes.

Fixes: #1814 (closed), #3242 (closed)