* Add modal.Wait() that creates a global progress bar modal which is not
dismissable by the user; the caller must Dismiss() the modal
themselves when ready.
* It will be useful in the future in case e.g. saving a Level needs to
take a while to rebalance chunks and the modal prevents ALL
interaction with the game so the user can't further modify the level
while it's busy refactoring itself.
* Cheat code: "test wait screen" to show the Wait modal for 10 seconds.
