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>

data/dbus-interfaces/org.gnome.Shell.Introspect.xml

@@ -18,6 +18,12 @@
     -->
     <signal name="RunningApplicationsChanged" />
 
+    <!--
+        WindowsChanged:
+        @short_description: Notifies when any window opens or closes
+    -->
+    <signal name="WindowsChanged" />
+
     <!--
         GetRunningApplications:
         @short_description: Retrieves the description of all running applications

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);