xactengine3_7: Implement IXACT3Engine::RegisterNotification and IXACT3Engine::UnRegisterNotification functions.

Based on patches from Ethan Lee and Alistair Leslie-Hughes.
Signed-off-by: Vijay Kiran Kamuju <infyquest@gmail.com>
Signed-off-by: Alistair Leslie-Hughes <leslie_alistair@hotmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>