Marble: Add a tag handler for Model and associated GeoData classes
completed by: Mayank Madan
mentors: Torsten Rahn, René Küttner, Dennis Nienhüser
Add GeoDataModel and getters/settsers for its properties as described in https://developers.google.com/kml/documentation/kmlreference#model. The location property should be a GeoDataCoordinates type.
Implement kml tag handler for Model and its children. Some child tags already have appropriate handlers. Extend them to check for Model as a parent.
Add a test case to check successful parsing.