GMenu: add g_menu_item_set_icon() convenience

This function takes a GIcon, serialises it and sets the resulting
GVariant as the "icon" attribute on the menu item.  We will need to add
a patch to Gtk to actually consume this icon.

Also add G_MENU_ATTRIBUTE_ICON.

https://bugzilla.gnome.org/show_bug.cgi?id=688820