Add hint for touch -> mouse event synthesizing

Add hint for touch -> mouse event synthesizing

Commit 7808ec79 changes QApplication to synthesize
mouse events from (unhandled) touch events.

On Mac OS X this creates a conflict for two-finger
scroll swipes, which generates both touch events and
mouse wheel events: scrolling in QTextEdit will also
select the text.

Add a SynthesizeMouseFromTouchEvents platform style
hint that enables the event synthesising. Set to true
by default and false in Cocoa.

Change-Id: I1ffa5a141476aa38b81ce92a87eff676c7ec2276
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>