It's now possible to add a GMenu as a submenu of another GMenu.
Simply use the GMenu::add_submenu(NonnullOwnPtr<GMenu>) API :^)
The WindowServer now keeps track of a stack of open menus rather than
just one "current menu". This code needs a bit more work, but the basic
functionality is now here!
Instead of LibGUI and WindowServer building their own copies of the drawing
and graphics code, let's it in a separate LibDraw library.
This avoids building the code twice, and will encourage better separation
of concerns. :^)
