Make orientable base classes instantiable

This commit adds constructors for GtkPaned, GtkBox, GtkButtonBox,
GtkRuler, GtkScale, GtkScrollbar and GtkSeparator and makes these
types instantiable.

gtk/gtkscrollbar.h

@@ -66,6 +66,8 @@ struct _GtkScrollbarClass
 
 
 GType       gtk_scrollbar_get_type (void) G_GNUC_CONST;
+GtkWidget * gtk_scrollbar_new      (GtkOrientation  orientation,
+                                    GtkAdjustment  *adjustment);
 
 G_END_DECLS