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/remoteSearch.js

@@ -204,7 +204,7 @@ var RemoteSearchProvider = class {
                                          g_interface_info: proxyInfo,
                                          g_interface_name: proxyInfo.name,
                                          gFlags });
-        this.proxy.init_async(GLib.PRIORITY_DEFAULT, null, null);
+        this.proxy.init_async(GLib.PRIORITY_DEFAULT, null);
 
         this.appInfo = appInfo;
         this.id = appInfo.get_id();