GDesktopAppInfo: Try to always correctly set id
Specs say that on Unix id should be desktop file id from the xdg menu specification, however, currently code just uses basename of .desktop file. Fix that by finding the .desktop file in all the desktop_file_dirs and use basename only as a fallback.
Signed-off-by: Ivaylo Dimitrov ivo.g.dimitrov.75@gmail.com