Tiled 1.2 Support

[Zulu-Inuoe]

Tiled 1.2 includes a few relatively substantial format changes:

• Layer objects now each have an ID
• The JSON Map format now stores the tiles in a tileset as an array of Tile objects, rather than a dictionary of ID->Tile Object values.
• Custom tile properties are now stored within each Tile instead of being included as tileproperties in the Tileset object.
• Custom properties are now stored in an array instead of an object where the property names were the keys. Each property is now an object that stores the name, type and value of the property. The separate propertytypes and tilepropertytypes attributes have been removed.

A few minor others, but at a minimum these'll need to be incorporated to be able to generally load Tiled 1.2 maps