Game only entities

[GSIO01]

Game only entities are entities that don't require to be send to the client and therefore only exit on the game side (similar to client entities/ local entities).
This would require to updates some of the functions that handle entities right now as the game only entiies would have an separate entity index.
Biggest pro of this is that this will help solving problems we have with the net code not beeing able to handle maps with many entities to well.

[GSIO01]

Youtrack Issue ID: RPGX-6