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
Expand and formalise the syntax for detailed action names, adding a
well-documented (and tested) public parser API for them.
Port the only GLib-based user of detailed action names to the new API:
g_menu_item_set_detailed_action(). The users in Gtk+ will also be
ported soon.
https://bugzilla.gnome.org/show_bug.cgi?id=688954
The code assumes in various places that ':' does not occur
in attribute names. We are a little more strict than that,
and only allow lowercase ASCII, digits and '-'.