js: Promisify async operations

Promises make asynchronous operations easier to manage, in particular
when used through the async/await syntax that allows for asynchronous
code to closely resemble synchronous one.

gjs has included a Gio._promisify() helper for a while now, which
monkey-patches methods that follow GIO's async pattern to return a
Promise when called without a callback argument.

Use that to get rid of all those GAsyncReadyCallbacks!

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1126

js/ui/shellDBus.js

@@ -325,8 +325,7 @@ var GnomeShellExtensions = class {
             null,
             Gio.DBusCallFlags.NONE,
             -1,
-            null,
-            (conn, res) => conn.call_finish(res));
+            null);
     }
 
     ReloadExtensionAsync(params, invocation) {