Improvements for TileMapObject

Here are some improvements for the TileMapObject : 
 - Tiles changes doesn't need a full regeneration of the TileMap de1408f
 - The selected tile ID is shown in the TileEditor (not in the TileSetPanel anymore) bbeeba2
 - Fix a bug with mono bitmaps in the TileEditor (already fixed in Tile(Set/Map)Panel) 581f2a4 #46 

**Changes inside GDCore :** PolygonCollision tests ignores polygons with less than 3 vertices (as TileMapObject now have "empty" polygon for empty tiles) de1408f