Add new STEP event and methods for creating/parsing the event
Update design docs.
Add new STEP_DONE message and method to create/parse.

API: GstEvent::gst_event_new_step()
API: GstEvent::gst_event_parse_step()
API: GstMessage::gst_message_new_step_done()
API: GstMessage::gst_message_parse_step_done()