Clean Workspace::updateXStackingOrder()

No need to create x_stacing by iterating over stacking_order, it can be
copied. We are not performing any operations over it.
Also no need to instanciate the tree until we want to use it.

layers.cpp

@@ -723,13 +723,11 @@ QList<Toplevel *> Workspace::xStackingOrder() const
 
 void Workspace::updateXStackingOrder()
 {
-    x_stacking.clear();
-    std::unique_ptr<Xcb::Tree> tree{std::move(m_xStackingQueryTree)};
     // use our own stacking order, not the X one, as they may differ
-    foreach (Toplevel * c, stacking_order)
-    x_stacking.append(c);
+    x_stacking = stacking_order;
 
-    if (tree && !tree->isNull()) {
+    if (m_xStackingQueryTree && !m_xStackingQueryTree->isNull()) {
+        std::unique_ptr<Xcb::Tree> tree{std::move(m_xStackingQueryTree)};
         xcb_window_t *windows = tree->children();
         const auto count = tree->data()->children_len;
         int foundUnmanagedCount = unmanaged.count();