Close a race condition that prevented switching spaces.

The problem seemed to be that the keyboard events and mouse events would happen
at different times. Ideally the order of events for moving a window to a space
would be

mouse down
keyboard down
keyboard up
space changes
mouse up

but it seems like occasionally it would be

mouse down
keyboard down
keyboard up
mouse up
space changes

So the window wouldn't get dragged to the subsequent space. Simply putting the
mouse up on a half second delay should greatly improve things.

Should fix #11