Fix for Final Boards lingering by using dispatch queue

[nheagy]

Working from the theory that the bug is caused because the mouseExisted event fires before the mouseEntered event has displayed the panel, this patch ensures that Final Boards dismiss by putting their creation inside a DispatchWorkItem that is then cancelled in the exit method.

We can still speed up creation of the final board panel in a few ways - one being building for arm64 on Apple Silicon - but the race condition would still remain without this PR.