qt/qml: filter mouse events in Flickable and implement deselection

After this merge request, it is not possible to drag scroll Flickable using mouse (this is probably not wanted feature anyway). Touch behavior should be retained.

It works by filtering non synthesized mouse events by installing an event filter.

• Fixes #25879 (closed).
• Makes it possible to implement rubber band selection.
• Right mouse interactions may generate QContextMenuEvent instead of mouse events depending on the platform. This event is not respected in Qt Quick side and this should be corrected. This is trivial to correct in custom compositors, but not in "dummy" because QQuickView does not provide QWidget interface. The workaround used is to anchor MouseArea to the view so that it captures right button events.