WindowServer: Allow windows to be pinnable (always on top)

This patch adds the concept of a window being "Pinnable" (always drawn
on top of other windows). This can be toggled through a new checkable
action in the top left corner's window menu.

Userland/Services/WindowServer/WindowStack.h

@@ -22,6 +22,7 @@ public:
     void add_to_back(Window&);
     void remove(Window&);
     void move_to_front(Window&);
+    void move_pinned_windows_to_front();
 
     enum class MoveAllWindowsTo {
         Front,