window-list: Handle changes to workspace menu

For now the menu is always set at construction time, however this
will change in the future. Prepare for that by handling the
`menu-set` signal, similar to the top bar.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/307>

extensions/window-list/extension.js

@@ -765,7 +765,9 @@ class WindowList extends St.Widget {
         this._updateWorkspaceIndicatorVisibility();
 
         this._menuManager = new PopupMenu.PopupMenuManager(this);
-        this._menuManager.addMenu(this._workspaceIndicator.menu);
+        this._workspaceIndicator.connectObject('menu-set',
+            () => this._onWorkspaceMenuSet(), this);
+        this._onWorkspaceMenuSet();
 
         Main.layoutManager.addChrome(this, {
             affectsStruts: true,
@@ -862,6 +864,11 @@ class WindowList extends St.Widget {
         children[newActive].activate();
     }
 
+    _onWorkspaceMenuSet() {
+        if (this._workspaceIndicator.menu)
+            this._menuManager.addMenu(this._workspaceIndicator.menu);
+    }
+
     _updatePosition() {
         this.set_position(
             this._monitor.x,