Calling screen maximize while dragging could lead to UI layout change
which affects context, this lead to crashes in the icon drawing and
selection code. This patch prevents maximize operator from running if
dragging is in progress.
Might need to look into why `drag->imb->rect` is None immediately after
calling maximize, we might want it to work since it's sometimes
more convenient to drag then put into a big recieving box. But since
`wmDropBox` is predetermined, this can be a somewhat problematic.
YimingWu