Enable touch events on Mac for PinchArea

Enabling touch events on a window causes scroll event
lag so we want to avoid avoid it as far as possible.

Enable/disable on scene changes, similar to what we do with
WA_AcceptTouchEvents for widgets, and in change
I2e5b5e2b093cccfc5253f7228f5ec0c588c60371 for MultiPointTouchArea.

Change-Id: I8cd8d172ffd93cfc4ec115917cc8662202f3b069
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>