Previously QToolButtonExt had some undesirable edge case behaviour.

Firstly, once the user presses down on a QToolButtonExt there is no
way to cancel the action; either a short or a long click will be
generated. Compare this to a normal button, which can be cancelled by
releasing the mouse outside of the button area.

Secondly, with the mouse button held down, moving the mouse in and out
of the button area will generate multiple short click events, when no
event at all is desired.

This patch corrects this: by releasing the mouse outside the button area
no event is generated; additionally no short click events are generated
simply by moving the depressed mouse in and out of the button.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>