introspect: Add GetWindows method

The `GetWindows` method gives access to the list of windows for each
application with some of their properties, so utilities such as dogtail
can pick the window of their choice to interfere with using the provided
window id.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/326

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

@@ -33,5 +33,29 @@
     <method name="GetRunningApplications">
       <arg name="apps" direction="out" type="a{sa{sv}}" />
     </method>
+
+    <!--
+        GetWindows:
+        @short_description: Retrieves the current list of windows and their properties
+
+        A window is exposed as:
+        * t ID: unique ID of the window
+        * a{sv} properties: high-level properties
+
+          Known properties:
+
+          - "title"       (s): (readonly) title of the window
+          - "app-id"      (s): (readonly) application ID of the window
+          - "wm-class"    (s): (readonly) class of the window
+          - "client-type" (u): (readonly) 0 for Wayland, 1 for X11
+          - "is-hidden"   (b): (readonly) if the window is currently hidden
+          - "has-focus"   (b): (readonly) if the window currently have
+                                          keyboard focus
+          - "width"       (u): (readonly) width of the window
+          - "height"      (u): (readonly) height of the window
+    -->
+    <method name="GetWindows">
+      <arg name="windows" direction="out" type="a{ta{sv}}" />
+    </method>
   </interface>
 </node>