app, libgimp*, modules: don't use g_type_class_add_private() ...

... and G_TYPE_INSTANCE_GET_PRIVATE()

g_type_class_add_private() and G_TYPE_INSTANCE_GET_PRIVATE() were
deprecated in GLib 2.58.  Instead, use
G_DEFINE_[ABSTRACT_]TYPE_WITH_PRIVATE(), and
G_ADD_PRIVATE[_DYNAMIC](), and the implictly-defined
foo_get_instance_private() functions, all of which are available in
the GLib versions we depend on.

This commit only covers types registered using one of the
G_DEFINE_FOO() macros (i.e., most types), but not types with a
custom registration function, of which we still have a few -- GLib
currently only provides a (non-deprecated) public API for adding a
private struct using the G_DEFINE_FOO() macros.

Note that this commit was 99% auto-generated (because I'm not
*that* crazy :), so if there are any style mismatches... we'll have
to live with them for now.

app/display/gimptoolwidget.c

@@ -96,7 +96,7 @@ static gboolean gimp_tool_widget_real_key_press     (GimpToolWidget  *widget,
                                                      GdkEventKey     *kevent);
 
 
-G_DEFINE_TYPE (GimpToolWidget, gimp_tool_widget, GIMP_TYPE_OBJECT)
+G_DEFINE_TYPE_WITH_PRIVATE (GimpToolWidget, gimp_tool_widget, GIMP_TYPE_OBJECT)
 
 #define parent_class gimp_tool_widget_parent_class
 
@@ -194,16 +194,12 @@ gimp_tool_widget_class_init (GimpToolWidgetClass *klass)
                                                         NULL, NULL,
                                                         GIMP_TYPE_CANVAS_ITEM,
                                                         GIMP_PARAM_READABLE));
-
-  g_type_class_add_private (klass, sizeof (GimpToolWidgetPrivate));
 }
 
 static void
 gimp_tool_widget_init (GimpToolWidget *widget)
 {
-  widget->private = G_TYPE_INSTANCE_GET_PRIVATE (widget,
-                                                 GIMP_TYPE_TOOL_WIDGET,
-                                                 GimpToolWidgetPrivate);
+  widget->private = gimp_tool_widget_get_instance_private (widget);
 }
 
 static void