Make the file transfer portal setup sync again

We need to register the portal mime types before
the others to prefer them, doing this call async
messes up that ordering.

This is effectively reverting 69fb3648b2

gdk/filetransferportal.c

@@ -480,20 +480,8 @@ connection_closed (GDBusConnection *connection,
 }
 
 static void
-got_proxy (GObject *source,
-           GAsyncResult *result,
-           gpointer data)
+finish_registration (void)
 {
-  GError *error = NULL;
-
-  file_transfer_proxy = g_dbus_proxy_new_for_bus_finish (result, &error);
-  if (!file_transfer_proxy)
-    {
-      g_message ("Failed to get file transfer portal: %s", error->message);
-      g_clear_error (&error);
-      return;
-    }
-
   gdk_content_register_serializer (G_TYPE_FILE,
                                    "application/vnd.portal.files",
                                    portal_file_serializer,
@@ -531,7 +519,8 @@ file_transfer_portal_register (void)
   if (!called)
     {
       called = TRUE;
-      g_dbus_proxy_new_for_bus (G_BUS_TYPE_SESSION,
+
+      file_transfer_proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SESSION,
                                 G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES
                                 | G_DBUS_PROXY_FLAGS_DO_NOT_CONNECT_SIGNALS
                                 | G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START,
@@ -540,8 +529,9 @@ file_transfer_portal_register (void)
                                 "/org/freedesktop/portal/documents",
                                 "org.freedesktop.portal.FileTransfer",
                                 NULL,
-                                got_proxy,
                                 NULL);
+      if (file_transfer_proxy)
+        finish_registration ();
     }
 }