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/widgets/gimpspinscale.c

@@ -82,9 +82,7 @@ struct _GimpSpinScalePrivate
   gint             pointer_warp_start_x;
 };
 
-#define GET_PRIVATE(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), \
-                                                       GIMP_TYPE_SPIN_SCALE, \
-                                                       GimpSpinScalePrivate))
+#define GET_PRIVATE(obj) ((GimpSpinScalePrivate *) gimp_spin_scale_get_instance_private ((GimpSpinScale *) (obj)))
 
 
 static void       gimp_spin_scale_dispose           (GObject          *object);
@@ -129,7 +127,8 @@ static void       gimp_spin_scale_setup_mnemonic    (GimpSpinScale    *scale,
                                                      guint             previous_keyval);
 
 
-G_DEFINE_TYPE (GimpSpinScale, gimp_spin_scale, GTK_TYPE_SPIN_BUTTON);
+G_DEFINE_TYPE_WITH_PRIVATE (GimpSpinScale, gimp_spin_scale,
+                            GTK_TYPE_SPIN_BUTTON)
 
 #define parent_class gimp_spin_scale_parent_class
 
@@ -162,8 +161,6 @@ gimp_spin_scale_class_init (GimpSpinScaleClass *klass)
                                    g_param_spec_string ("label", NULL, NULL,
                                                         NULL,
                                                         GIMP_PARAM_READWRITE));
-
-  g_type_class_add_private (klass, sizeof (GimpSpinScalePrivate));
 }
 
 static void