Added Tile Grid (X-UniTMX)

Added Tile Grid with X-UniTMX. Every tile object on the map is generated
during runtime from an XML file generated by Tiled.

Each Tile has its own Tile instance. You can access any Tile in the main
tile grid by doing GameMap.Instance.mainGrid[x, y]. You also access the
Map instance which contains a lot of other useful functionality, like
the ability to convert from world point to tile grid indices & vice
versa, etc. See the source code.