Use gtk_widget_set_visible()

In places where the pattern

  if (show)
    gtk_widget_show (widget);
  else
    gtk_widget_hide (widget);

is used, change to

  gtk_widget_set_visible (widget, show);

Also do some other minor cleanups.

app/widgets/gimpwidgets-utils.c

@@ -1002,10 +1002,8 @@ gimp_toggle_button_set_visible (GtkToggleButton *toggle,
   g_return_if_fail (GTK_IS_TOGGLE_BUTTON (toggle));
   g_return_if_fail (GTK_IS_WIDGET (widget));
 
-  if (gtk_toggle_button_get_active (toggle))
-    gtk_widget_show (widget);
-  else
-    gtk_widget_hide (widget);
+  gtk_widget_set_visible (widget,
+                          gtk_toggle_button_get_active (toggle));
 }
 
 static gboolean