Welcome MapThemeManager, welcome GeoSceneDocument!

This is going to replace the MapTheme-kludge now and furthermore preparing for the new layer management

- Adding MapThemeManager stub to ControlView
- Parsing a dummy DGML2 file in the MarbleModel::setMapTheme() method as an initial introduction of the whole GeoScene model

Adding a test dgml file. This needs to get renamed from .dgm2 to .dgml later on again once everything works as expected.