Deprecate g_type_class_add_private()

It's been 4 years and 8 development cycles since we introduced
G_ADD_PRIVATE and offset-based private data access. It is now
time to finally deprecate the old mechanism.

Closes: #699

gobject/gtype.c

@@ -76,7 +76,7 @@
  *
  * Type instance and class structs are limited to a total of 64 KiB,
  * including all parent types. Similarly, type instances' private data
- * (as created by g_type_class_add_private()) are limited to a total of
+ * (as created by G_ADD_PRIVATE()) are limited to a total of
  * 64 KiB. If a type instance needs a large static buffer, allocate it
  * separately (typically by using #GArray or #GPtrArray) and put a pointer
  * to the buffer in the structure.
@@ -4562,6 +4562,8 @@ gobject_init_ctor (void)
  * ]|
  *
  * Since: 2.4
+ * Deprecated: 2.58: Use the G_ADD_PRIVATE() macro with the `G_DEFINE_*`
+ *   family of macros to add instance private data to a type
  */
 void
 g_type_class_add_private (gpointer g_class,

gobject/gtype.h

@@ -634,6 +634,8 @@ struct _GTypeQuery
  * This macro should only be used in type implementations.
  * 
  * Since: 2.4
+ * Deprecated: 2.58: Use %G_ADD_PRIVATE and the generated
+ *   `your_type_get_instance_private()` function instead
  * Returns: (not nullable): a pointer to the private data structure
  */
 #define G_TYPE_INSTANCE_GET_PRIVATE(instance, g_type, c_type)   ((c_type*) g_type_instance_get_private ((GTypeInstance*) (instance), (g_type)))
@@ -1297,7 +1299,7 @@ void  g_type_interface_add_prerequisite (GType			     interface_type,
 GLIB_AVAILABLE_IN_ALL
 GType*g_type_interface_prerequisites    (GType                       interface_type,
 					 guint                      *n_prerequisites);
-GLIB_AVAILABLE_IN_ALL
+GLIB_DEPRECATED_IN_2_58
 void     g_type_class_add_private       (gpointer                    g_class,
                                          gsize                       private_size);
 GLIB_AVAILABLE_IN_2_38

gobject/tests/private.c

@@ -128,7 +128,9 @@ G_DEFINE_TYPE (TestMixed, test_mixed, test_object_get_type ())
 static void
 test_mixed_class_init (TestMixedClass *klass)
 {
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
   g_type_class_add_private (klass, sizeof (TestMixedPrivate));
+G_GNUC_END_IGNORE_DEPRECATIONS
 }
 
 static void