WindowServer: Add WindowStack concept

This patch moves the window stack out of WindowManager and into its own
WindowStack class.

A WindowStack is an ordered list of windows with an optional highlight
window. The highlight window mechanism is used during Super+Tab window
switching to temporarily bring a window to the front.

This is mostly mechanical, just moving the code to its own class.

Userland/Services/WindowServer/WindowStack.cpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "WindowStack.h"
+
+namespace WindowServer {
+
+WindowStack::WindowStack()
+{
+}
+
+WindowStack::~WindowStack()
+{
+}
+
+void WindowStack::add(Window& window)
+{
+    m_windows.append(window);
+}
+
+void WindowStack::remove(Window& window)
+{
+    m_windows.remove(window);
+}
+
+void WindowStack::move_to_front(Window& window)
+{
+    if (m_windows.last() != &window)
+        window.invalidate();
+    m_windows.remove(window);
+    m_windows.append(window);
+}
+
+void WindowStack::set_highlight_window(Window* window)
+{
+    if (!window)
+        m_highlight_window = nullptr;
+    else
+        m_highlight_window = window->make_weak_ptr<Window>();
+}
+
+}