LibGUI, WindowServer: Greatly simplify menubar logic

Currently, any number of menubars can be plugged in and out of a window.
This is unnecessary complexity, since we only need one menubar on a
window. This commit removes most of the logic for dynamically attaching
and detaching menubars and makes one menubar always available. The
menubar is only considered existent if it has at least a single menu in
it (in other words, an empty menubar will not be shown).

This commit additionally fixes a bug wherein menus added after a menubar
has been attached would not have their rects properly setup, and would
therefore appear glitched out on the top left corner of the menubar.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -59,7 +59,6 @@ set(SOURCES
     LinkLabel.cpp
     ListView.cpp
     Menu.cpp
-    Menubar.cpp
     MenuItem.cpp
     MessageBox.cpp
     Model.cpp