gicon: Add introspection for to_tokens / from_tokens vfunc's

The GIconIface virtual functions were not introspectable as they use
complex parameters that GI isn't able to compute alone.

So provide introspection metadata to the two function pointers
definitions.

GIcon::from_tokens is a static virtual function so it won't actually
work until GI support for it [1] is merged.

[1] https://gitlab.gnome.org/GNOME/gobject-introspection/-/merge_requests/361

gio/gicon.h

@@ -69,9 +69,38 @@ struct _GIconIface
   guint       (* hash)        (GIcon   *icon);
   gboolean    (* equal)       (GIcon   *icon1,
                                GIcon   *icon2);
+
+  /**
+   * GIconIface::to_tokens:
+   * @icon: The #GIcon
+   * @tokens: (element-type utf8) (out caller-allocates):
+   *   The array to fill with tokens
+   * @out_version: (out): version of serialized tokens
+   *
+   * Serializes the @icon into string tokens.
+   * This is can be invoked when g_icon_new_for_string() is called.
+   *
+   * Returns: %TRUE if serialization took place, %FALSE otherwise
+   *
+   * Since: 2.20
+   */
   gboolean    (* to_tokens)   (GIcon   *icon,
-			       GPtrArray *tokens,
+                               GPtrArray *tokens,
                                gint    *out_version);
+
+  /**
+   * GIconIface::from_tokens:
+   * @tokens: (array length=num_tokens): An array of tokens
+   * @num_tokens: The number of tokens in @tokens
+   * @version: Version of the serialized tokens
+   * @error: Return location for errors, or %NULL to ignore
+   *
+   * Constructs a #GIcon from a list of @tokens.
+   *
+   * Returns: (nullable) (transfer full): the #GIcon or %NULL on error
+   *
+   * Since: 2.20
+   */
   GIcon *     (* from_tokens) (gchar  **tokens,
                                gint     num_tokens,
                                gint     version,