Add support for QML QGeoMapObjects

Basically it should work like the QGeoMapPixmapObject only that instead of a pixmap we supply a QML component. That way it would be possible to get fancy POI markers that, for example, smoothly expand once tapped by the user.

I think usually the map object should simply be pinned to a certain coordinate and should not be affected by zoom. However an option to make it react to the current zoom level would be nice as well.