Added ability for "replace" map objects to replace groups of map objects

Added ability for "replace" map objects to replace groups of map objects by others.
It applies for instances of the MapObject and MonsterSpawnArea that are
fully contained within the replace area.
When MonsterSpawnAreas are impacted, the MapObjectReplace.strategy
variable determines whether removed spawn areas should see their monster
vanish or not, and whether newly introduced spawn areas should be fully
spawned at once or not.
debugmap was modified to test and give an example of behavior & map
editing.