Add external (X-*) keys of an app's desktop file as metadata AsApp
Add external (X-*) keys of an app's desktop file as metadata AsApp

This is helpful for use-cases where those keys are needed by users of
the library.

hughsie/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib