gaction: add parser for detailed action names

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

gio/gaction.h

@@ -81,6 +81,12 @@ void                    g_action_change_state                           (GAction
 GLIB_AVAILABLE_IN_ALL
 void                    g_action_activate                               (GAction            *action,
                                                                          GVariant           *parameter);
+
+GLIB_AVAILABLE_IN_2_38
+gboolean                g_action_parse_detailed_name                    (const gchar        *detailed_name,
+                                                                         gchar             **action_name,
+                                                                         GVariant          **target_value,
+                                                                         GError            **error);
 G_END_DECLS
 
 #endif /* __G_ACTION_H__ */