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/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     Window.cpp
     WindowFrame.cpp
     WindowManager.cpp
+    WindowStack.cpp
     WindowSwitcher.cpp
     WindowServerEndpoint.h
     WindowClientEndpoint.h