Use Gio.AppInfo.launch_default_for_uri instead of calling xdg-open
As explained in this [thread](https://discourse.gnome.org/t/do-not-show-open-with-dialog-when-running-xdg-open-from-a-flatpakd-app/1029/2) it is better to use the Gio functions than starting a subprocess. Also even if Flatpak provide xdg-open wrapper, it is better to have GLib manage the flatpak case directly.
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