Fixes rare crash on base facility dismantle

Deleting a base facility at x=2 y=3, so roughly where the ok button is
in the confirm deletion box, can occasionally cause a viewMouseOver
event to fire where the facility just was.  Due to mouse being moved at the same time as the ok button is clicked

This fix temporarily marks the facility as being dismantled so any mouse overring, in the tiny amount of time the game is fulfilling the player's request, does not cause the facility text to be mis-set.