MultiPointTouchArea eats mouse events, needs to ignore them

MouseArea ignores touches, MultiPointTouchArea needs to ignore mouse events.

Currently the input choice for QML is MouseArea or
MultiPointTouchArea. If the program expects only one of mouse or
touch events the choice is clear. If the program needs to accept
both, the MouseArea works if the interaction is limited to one touch
or button provided Qt::AA_SynthesizeMouseForUnhandledTouchEvents is
enabled (which by default is), but for multitouch interaction one
MouseArea accepting input blocks all other MouseAreas, and it can't
accept multiple in one area. If the program uses MultiPointTouchArea
instead, touch events are sent to QML, but mouse events were blocked
preventing any interaction from mouse events. This leaves a QML
application without any way to get both the full mouse (such as middle and right clicks) or multiple touch event data.