TileMapObject

Here is the first version of the TileMapObject.
It contains all the basic features like tilemap edition, predefined collision masks, changing tiles/texture at runtime.

I've just changed some files outside the Extensions/TileMapObject folder : 
 - I've added GD_API to the PolygonCollision function (to be able to use it in the extension)
 - Of course, I've added a line in the CMakeLists.txt to compile the extension

If you have some suggestions, I can still make some commits before you merge it.