This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
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!
