REG 5.15->6.0: when PinchHandler and DragHandler are used together, trackpad pinch gesture causes a jump

This can be reproduced on macOS with tests/manual/pinter/pinchHandler.qml for example: do a trackpad pinch on the blue rectangle, and it jumps to align its upper-left corner to the cursor position rather than rotating and scaling around the cursor position.

The reason seems to be that QQuickMultiPointHandler::wantsPointerEvent() returns true for all gesture events, but only PinchHandler actually knows how to handle them so far. DragHandler must not say that it "wants" them, because then it will receive them and react.

Later on, DragHandler probably could handle the PanNativeGesture. But that's not implemented yet, so for now it needs to ignore all gesture events.