decide whether DragHandler needs a filterChildren property and ability to filter childrens' events

MouseArea has drag.filterChildren. This would be useful in situations such as in https://github.com/johanhelsing/qt-csd-demo/blob/master/main.qml#L32 or in FakeFlickable: you might want to be able to drag something even if you pressed on an Item which has taken the exclusive grab (such as a Controls 2 Button).