Use the element name menu for the main node, and use two subnodes
with name arrow and style classes .top and .bottom for the arrows
of scrolling menus.
The GtkSubmenuDirection and GtkSubmenuPlacement enumerations
have been deprecated as public API for a while, but are still used
internally in the menu code. Move them to a private header. This
also prevents to generation of GObject boilerplate for these enums.