This is pretty much complete rewrite, Alt now only show tabs toolbar,
but when you press Alt again while also hovering tabs toolbar, then
menubar items will be shown and window controls likewise.
There's a weird behavior, some 1px slice that hides tabs toolbar when
cursor is moved from tabs to window controls, but doesn't trigger from
other direction mysteriously. But window controls can be accessed when
cursor is already where window controls would appear when Alt is pressed
the second time.
