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

subprojects/extensions-app/js/main.js

@@ -201,10 +201,7 @@ var ExtensionsWindow = GObject.registerClass({
             null,
             Gio.DBusCallFlags.NONE,
             -1,
-            null,
-            (o, res) => {
-                o.call_finish(res);
-            });
+            null);
     }
 
     _sortList(row1, row2) {