The original behaviour was a bit frustrating because it used the first
small mouse move to deduce the user's wished direction, but that is a bit
imprecise.
This patch divides the window in 9 rectangles:
- 4 for corners, used for diagonal resizing
- 4 for middles of sides, for resizing Up, Down, Left and Right
- 1 useless in the middle of the the window, which falls back on diagonal
resizing
This leads to a more predictive behaviour.
