I honestly don't believe text objects look good on map, and in real maps, I think text should be written on tiles.
However, for a variety of use cases (like in test maps in the /maps/test directory, it can be useful to be
able to display some text easily on a map.
This PR adds the ability to display this text.
Note: the "font" support cannot work correctly, as Tiled is listing fonts from the local
system, and those fonts are not available in a browser.
This commit adds a listener in the preload function that will be triggered as soon as the map is loaded.
This function will load the resources from the map (tilesets) defined in the map.
That way, we don't have to define manually the list of tiles that have to be loaded (at the expense of a slight delay in loading since we must wait for the map to be loaded to start loading the tiles).
