PopupMenu: introduce PopupMenuSection

PopupMenu: introduce PopupMenuSection

Complex popup menus require the ability to manager sequences of items
as "sections", to which you can add and and remove items, as well
as hide and show.
PopupMenuSection does exactly that, leveraging the existing machinery
for submenus, but without being exposed as a submenu to the user.
Also, make getMenuItems() private, since it is used for different things
now and may change semantics in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=621707