make it easier to load qml configuration files from the filesystem without full path

$ qml --list-conf
Built-in configurations:
  default
  resizeToItem

It does not list configuration files from the filesystem, even though the user can write custom config files. And the built-in configs can be chosen with single-word names, whereas to use a custom config file you have to give the full path. So I think we should also detect configurations that are installed in a standard location, probably in individual directories under StandardPaths.AppConfigLocation, list them by the directory name there, and make it work with qml -c myconfig just as you can do qml -c resizeToItem.