Added saving monster to the overmap.

The interface is in overmapbuffer, so it's global.
The function overmapbuffer::despawn_monster replaces game::force_save_monster.
Stored monsters are loaded from overmap in map::spawn_monsters, like the previous implementation and like the static spawn points.
All monsters are stored on the overmap as exact copies, no monster goes back into a group.

Removed monster being stored in the submaps.