This patch begins the transition away from the global menu towards
per-window menus instead.
The global menu looks neat, but has always felt clunky, and there
are a number of usability problems with it, especially in programs
with multiple windows.
You can now call GUI::Window::set_menubar() to add a menubar to
your window. It will be specific to that one window only.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
Also, only mark the menu bar item as opened if a menu was actually
opened through the menu bar.
These changes allow a menu to be used both in the menu bar as well
as a context menu.
Fixes#5469
