If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.
It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.
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 '-'.