make it possible to press a TapHandler and a MouseArea at the same time

This is a bit like the need for interoperability with QtQuick Controls: TapHandler may be used to augment behavior of something else. Of course when gesturePolicy is set in such a way that TapHandler does not take the exclusive grab, MouseArea can still get it.

TapHandler might be a child of MouseArea, or a child of its parent, or simply on top in Z order. We just need to think about how to get more control over what happens in those cases.

One pattern that has been suggested before is forwardTo, like the Keys handler can do. Not sure if it's the cleanest way we can come up with.

(This is from the KDE Plasma project; the actual use cases need a bit of clarification.)