introspect: Add WindowsChanged signal

The screencast portal supports recording a single window,
and presents a list of open windows when that option is
selected. To allow updating that list when windows are
opened or closed, add a new "WindowsChanged" signal that
the portal can listen to.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2229>

js/misc/introspect.js

@@ -42,6 +42,9 @@ var IntrospectService = class {
                             this._syncRunningApplications();
                         });
 
+        tracker.connect('tracked-windows-changed',
+            () => this._dbusImpl.emit_signal('WindowsChanged', null));
+
         this._syncRunningApplications();
 
         this._senderChecker = new DBusSenderChecker(APP_ALLOWLIST);