Save 'isActive' for MapObjects & SpawnArea in savegames

* Stores the "isActive" boolean in savegames for map objects and spawn areas
* Adds functions in MapController to activate and deactivate MapObject areas
* Adds functions in MonsterSpawnController to activate and deactivate MonsterSpawnAreas
* When a spawn area is deactivated, all monsters should be removed, since the spawn area is considered to be non-existant.
* Adds boolean "isActiveWhenStartingGame" to map objects and spawn areas, that specify what "isActive" should be set to when starting a game. (Initially, all of these are true)
* This commit does not add any way to actually activate or deactive areas. That can come as a later commit (possibly using script effects to do that).