textview: Use standard padding

Given that GtkSourceView exists, we will have
to keep GtkTextView subclassable until a replacement
appears.

gtk/gtktextview.h

@@ -189,11 +189,7 @@ struct _GtkTextViewClass
 
   /*< private >*/
 
-  /* Padding for future expansion */
-  void (*_gtk_reserved1) (void);
-  void (*_gtk_reserved2) (void);
-  void (*_gtk_reserved3) (void);
-  void (*_gtk_reserved4) (void);
+  gpointer padding[8];
 };
 
 GDK_AVAILABLE_IN_ALL