`FirstGlobalIdentifier` is only used with the map to reference the tileset which contains the tile referenced. It isn't really part of the definition of a `TiledMapTileset` even though the `.TMX` format includes it with the `<tileset>` tag
Pulled out the methods for writing and reading the content files for tilesets into their own classes to allow loading tilesets independently from the maps.
Changed the reference of tileset textures so the `Texture2DContent` isn't referenced, but built into the file as tilesets don't reference the same texture more than once.
Adds the `Content` field to `TiledMapImageContent` to reference the `Texture2DContent` instead of manually loading the content with the `ContentManager`.
stefanrbk