allow MouseArea to directly handle touch events

Similar to QTBUG-22056 and QTBUG-30840 and QTBUG-31047: the goal is that every Item which can handle mouse events can also handle touch events, and vice-versa, so that synthesizing one type of event from another becomes deprecated (or at least avoidable and unnecessary) over time. It will reduce bugs and also allow touching multiple MouseAreas at the same time, which is necessary for multi-user touch applications and even certain single-user applications, such as a virtual keyboard, a piano or a sound mixer (multiple Sliders in parallel). The reason that you cannot currently do that is because Qt has a deep long-standing assumption that there is only one mouse.