Use def_* macros for automatic map event counts

Resolves #742

engine/overworld/player_object.asm

@@ -55,6 +55,7 @@ PlayerObjectTemplate:
 ; A dummy map object used to initialize the player object.
 ; Shorter than the actual amount copied by two bytes.
 ; Said bytes seem to be unused.
+_NUM_OBJECT_EVENTS = 0
 	object_event -4, -4, SPRITE_CHRIS, SPRITEMOVEDATA_PLAYER, 15, 15, -1, -1, 0, OBJECTTYPE_SCRIPT, 0, 0, -1
 
 CopyDECoordsToMapObject::