Allow monsters to be added in the map editor

[stephen]

Guarded exit segments contain lots of initial vampire bats to populate the guarded exit point. This is currently implemented by a hack: the floor tile with little scratches on it represents a location where a bat should be placed initially.

Should get rid of this and allow the map editor to directly place initial monsters within the dungeon. This will not only make the code cleaner but it will also be useful for the Ogre quest (#137) where we need to place an ogre from inside the map editor.