Added a Sprite.tileManagementPlugin property for plugins that want to
replace the standard tilemap/tileset interface. This includes a new
external file field in .aseprite files to specify that the sprite
tiles are controlled by a specific plugin.
Once this property is set, the standard tilemap/tileset modes
selectors will disappear and the only way to make then available will
be setting this property to nil/empty string again.
Fix https://github.com/aseprite/Attachment-System/issues/21
