libgimpwidgets: add gtk_scale_new() to gimp3migration.[ch]

and use it all over the place. Also change some GtkObject* variables
to GtkAdjustment*.

libgimpwidgets/gimp3migration.c

@@ -47,6 +47,16 @@ gtk_button_box_new (GtkOrientation  orientation)
     return gtk_vbutton_box_new ();
 }
 
+GtkWidget *
+gtk_scale_new (GtkOrientation  orientation,
+               GtkAdjustment  *adjustment)
+{
+  if (orientation == GTK_ORIENTATION_HORIZONTAL)
+    return gtk_hscale_new (adjustment);
+  else
+    return gtk_vscale_new (adjustment);
+}
+
 GtkWidget *
 gtk_scrollbar_new (GtkOrientation  orientation,
                    GtkAdjustment  *adjustment)