patch 8.0.0521: GtkForm handling is outdated

Problem:    GtkForm handling is outdated.
Solution:   Get rid of event filter functions.  Get rid of GtkForm.width and
            .height.  Eliminate gtk_widget_size_request() calls. (Kazunobu
            Kuriyama)

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    521,
 /**/
     520,
 /**/