fix [#33466] alt + mouse wheel

Alt+Mouse Wheel events depended on the button under the mouse being active, else it would change the current frame.
Button re-activation wasn't 100% reliable since it depends on fake mousemove events.

Now keep button active when its executed if the mouse stays over it.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54929 954f8c5b-7b00-dc11-b283-0030488c597c