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

docs/reference/gio/gio-sections.txt

@@ -3800,6 +3800,7 @@ g_menu_item_new_from_model
 
 <SUBSECTION>
 g_menu_item_set_label
+g_menu_item_set_icon
 g_menu_item_set_action_and_target_value
 g_menu_item_set_action_and_target
 g_menu_item_set_detailed_action
@@ -3836,8 +3837,9 @@ g_menu_model_get_n_items
 
 <SUBSECTION>
 G_MENU_ATTRIBUTE_ACTION
-G_MENU_ATTRIBUTE_LABEL
 G_MENU_ATTRIBUTE_TARGET
+G_MENU_ATTRIBUTE_LABEL
+G_MENU_ATTRIBUTE_ICON
 G_MENU_LINK_SECTION
 G_MENU_LINK_SUBMENU
 

gio/gmenu.c

@@ -26,6 +26,8 @@
 #include "gaction.h"
 #include <string.h>
 
+#include "gicon.h"
+
 /**
  * SECTION:gmenu
  * @title: GMenu
@@ -1348,3 +1350,42 @@ g_menu_item_new_from_model (GMenuModel *model,
 
   return menu_item;
 }
+
+/**
+ * g_menu_item_set_icon:
+ * @menu_item: a #GMenuItem
+ * @icon: a #GIcon, or %NULL
+ *
+ * Sets (or unsets) the icon on @menu_item.
+ *
+ * This call is the same as calling g_icon_serialize() and using the
+ * result as the value to g_menu_item_set_attribute_value() for
+ * %G_MENU_ATTRIBUTE_ICON.
+ *
+ * This API is only intended for use with "noun" menu items; things like
+ * bookmarks or applications in an "Open With" menu.  Don't use it on
+ * menu items corresponding to verbs (eg: stock icons for 'Save' or
+ * 'Quit').
+ *
+ * If @icon is %NULL then the icon is unset.
+ *
+ * Since: 2.38
+ **/
+void
+g_menu_item_set_icon (GMenuItem *menu_item,
+                      GIcon     *icon)
+{
+  GVariant *value;
+
+  g_return_if_fail (G_IS_MENU_ITEM (menu_item));
+  g_return_if_fail (G_IS_ICON (icon));
+
+  if (icon != NULL)
+    value = g_icon_serialize (icon);
+  else
+    value = NULL;
+
+  g_menu_item_set_attribute_value (menu_item, G_MENU_ATTRIBUTE_ICON, value);
+  if (value)
+    g_variant_unref (value);
+}

gio/gmenu.h

@@ -175,6 +175,10 @@ GLIB_AVAILABLE_IN_2_32
 void        g_menu_item_set_detailed_action         (GMenuItem   *menu_item,
                                                      const gchar *detailed_action);
 
+GLIB_AVAILABLE_IN_2_38
+void        g_menu_item_set_icon                    (GMenuItem   *menu_item,
+                                                     GIcon       *icon);
+
 G_END_DECLS
 
 #endif /* __G_MENU_H__ */

gio/gmenumodel.h

@@ -73,6 +73,21 @@ G_BEGIN_DECLS
  **/
 #define G_MENU_ATTRIBUTE_LABEL "label"
 
+/**
+ * G_MENU_ATTRIBUTE_ICON:
+ *
+ * The menu item attribute which holds the icon of the item.
+ *
+ * The icon is stored in the format returned by g_icon_serialize().
+ *
+ * This attribute is intended only to represent 'noun' icons such as
+ * favicons for a webpage, or application icons.  It should not be used
+ * for 'verbs' (ie: stock icons).
+ *
+ * Since: 2.38
+ **/
+#define G_MENU_ATTRIBUTE_ICON "icon"
+
 /**
  * G_MENU_LINK_SUBMENU:
  *