When dragging with a large threshold (using a tablet for example),
it's possible to press another key before the drag threshold is reached.
So tweaking then pressing X would show the delete popup instead of
transforming along the X-axis.
Now key presses while dragging cause the drag event to be evaluated
before the key press.
Note that to properly base the mouse-move event on the previous
state the last handled event is now stored in the window.
Without this the inserted mouse-move event may contain invalid values
from the next event (it's modifier state or other `prev_*` values).
Requested by @JulienKaspar.
1f1dcf41d5