consider deprecating PinchArea

Of all the Areas, this is the perhaps the easiest one to replace with a pointer handler: PinchHandler. The feature set was restricted to begin with (QTBUG-57247 etc) so you gain functionality by switching. And we can treat this as a trial balloon for eventually deprecating all the Areas in favor of pointer handlers: I'm curious whether we will get bugs explaining reasons why someone needs a behavioral component to be a QQuickItem subclass.