Install all data files on XDG targets based on the configured AppID and report that value to the window system

This adheres to the standard naming conventions required in newer XDG specifications and is sadly enforced by Flatpak.

With this change, the default AppID for the launcher is org.solarus_games.solarus.Launcher and org.solarus_games.solarus.Runner for the game runner window, but forks could easily set their own. There is no technical reasons why these should be separate, I just thought it nicely represents the split in the application itself and it theoretically also enables the two applications to have different window icons on Wayland (where the window icon is determined by looking up the window's reported AppID in the system icon path).

If this is merged, the plan would be to add a similar change to the QuestEditor and have it report itself under the same org.solarus_games.solarus namespace.