notification: Add the desktop-file-id key for portal backends

This makes it possible for portal impl to properly look up a localized
name of the app. The app id that is sent as the first argument can
sometimes be mapped to the desktop file but this isn't generally true
and snap app ids for example cannot be used for this.

data/org.freedesktop.impl.portal.Notification.xml

@@ -47,6 +47,13 @@
 
         The format of the @notification is the same as for
         :ref:`org.freedesktop.portal.Notification.AddNotification`.
+        It additionally can contain the following keys:
+
+        * ``desktop-file-id`` (``s``)
+
+          The desktop file ID of the app adding the notification. A desktop
+          file ID is the basename of the desktop file, including the
+          .desktop extension.
 
         Since version 2, the icon property never uses the ``bytes`` option.
     -->

src/notification.c

@@ -1046,6 +1046,7 @@ handle_add_in_thread_func (GTask        *task,
 {
   CallData *call_data = task_data;
   GVariantBuilder builder;
+  const char *desktop_file_id;
   g_autoptr(GError) error = NULL;
 
   CALL_DATA_AUTOLOCK (call_data);
@@ -1077,6 +1078,10 @@ handle_add_in_thread_func (GTask        *task,
       return;
     }
 
+  desktop_file_id = xdp_app_info_get_desktop_file_id (call_data->app_info);
+  if (desktop_file_id)
+    g_variant_builder_add (&builder, "{ss}", "desktop-file-id", desktop_file_id);
+
   xdp_dbus_impl_notification_call_add_notification (impl,
                                                     xdp_app_info_get_id (call_data->app_info),
                                                     call_data->id,