[libtaskmanager] Notify when appmenu path changes

This isn't always static, espcially in GTK apps where the xproperty is
set later by gmenuproxyodel.

The Appmenu applet already watches for this role changing, but it wasn't
hooked up.

The wayland implementation does not have the same bug.

BUG: 422786

libtaskmanager/xwindowtasksmodel.cpp

@@ -398,6 +398,14 @@ void XWindowTasksModel::Private::windowChanged(WId window, NET::Properties prope
         changedRoles << Activities;
     }
 
+    if (properties2 & NET::WM2AppMenuServiceName) {
+        changedRoles << ApplicationMenuServiceName;
+    }
+
+    if (properties2 & NET::WM2AppMenuObjectPath) {
+        changedRoles << ApplicationMenuObjectPath;
+    }
+
     if (wipeInfoCache) {
         delete windowInfoCache.take(window);
     }