gstevent: add new api for parse seek event

Submitted by Jimmy Ohn

Description

generally, when we parse seek event using gst_event_parse_seek function, parameter is too many I think.
For example, If we just want to get the rate value from event, we should use like this gst_event_parse_seek(event, NULL, &rate, NULL, NULL, NULL...).
I agree this is minor problem but I think that If we use segment for parsing event, it would be better.

Version: 1.4.5