Improved semantics of some level editor menu saving/loading.
Started CampaignData and LevelData classes (in level_data.h) to make saving and loading more reusable.  The level editor will need this.
Added more scen version support for the LevelData loading.  Not tested!
Big messy mess.  Decoupled screen class from level editor data.  It took adding drawing data into LevelData, storing the obmap (spatial partition) pointer in each walker so it could point at LevelData::myobmap, making radar (minimap) work without the screen class, and making viewscreen drawing work with LevelData instead of myscreen.