Map references

[snipercup]

Requires #101

References are now set between a map and it's elements. Map elements include furniture, tiles and mobs

• When making changes to a map and saving your changes, the entities that were added will get a reference to the map added to their data. Entities that were removed from the map will have the reference to that map removed
• When furniture, tile or mob is deleted from the game data, all instances of it will be removed from each map in it's reference
• Added code for duplication of entities. That means that if you duplicate a furniture and it has an itemgroup set in it's container property, the itemgroup will get a reference to this newly duplicated furniture.

Example of furniture having a reference to a map called "udoad":

Example of a tile having a reference to a map called "udoad":