style: Handle sections in submenus

style: Handle sections in submenus

If a menu item in a submenu is part of a section, it should have
rounded bottom corners if both the item and the section are the
last child of its respective parent.

To express that, add a new .popup-menu-section class and use that
to undo/redo the rounding for items inside a section.

It would be possible to do without a new class with a selector like

 > StBoxLayout > .popup-menu-item:last-child:hover,
 :last-child > .popup-menu-item:last-child:hover

but that's hardly better with its heavy reliance on implementation
details.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4940

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2126>