tmxviewer can't view json-based maps

[eevee]

Just says

Error: Not a map file.

Looks like MapReader only reads XML. tmxrasterizer will happily render json maps, though, and even entire worlds.

[bjorn]

@eevee Indeed, I've changed it to use Tiled::readMap instead, and to call PluginManager::loadPlugins, such that now also JSON maps can be viewed.

The viewer is still very limited though, for example it can't even zoom in/out. But maybe it helps. :-)