Remove mouse grabber if touch used for mouse emulation has been grabbed

If an item grabs a touch that is currently being used for mouse pointer
emulation, the current mouse grabber should lose the mouse as mouse
events will no longer be generated from that touch point.

Example of what happens without this patch:
-User touches a MouseArea. It gets pressed.
-Some other item grabs that touch.
-This touch eventually ends.
-The MouseArea would still be pressed.
And what will happens instead after this patch:
-User touches a MouseArea. It gets pressed.
-Some other item grabs that touch.
-The MouseArea gets canceled and released
-This touch eventually ends.

Change-Id: I24114f18be564553a4a768243bb20379affe7a8f
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>