Allow opening a popup menu from another menu. While the child menu
is open, events on the parent menu are blocked. The parent menu
is kept open when the child menu is closed; the child menu on the
other hand is closed with the parent, e.g. when the focus moves
to another toplevel menu.
This feature will be used to implement combo box menu items.
https://bugzilla.gnome.org/show_bug.cgi?id=652837
5d2b7e2c9e