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.
