Vertexes are now internally C++ objects

Replaced GameMap vertexes with a MapObjectList<Vertex>.

Todo: DMU needs to be updated to work with de::MapObject rather than
runtime_mapdata_header_t.
Adjusted client/server headers for better decoupling
Sectors are now internally C++ objects
Allocate GameMap as a C++ instance

Constructor initializes everything to zero.
Removed the old runtime_mapdata_header_t from sector_s