Added segment helper structure and methods. Not fully implemented yet.

Added segment helper structure and methods. Not fully implemented yet.

Original commit message from CVS:
* check/Makefile.am:
* check/gst/gstsegment.c: (GST_START_TEST), (gstevents_suite),
(main):
* gst/Makefile.am:
* gst/gst.h:
* gst/gstsegment.c: (gst_segment_init), (gst_segment_set_duration),
(gst_segment_set_seek), (gst_segment_set_newsegment),
(gst_segment_to_stream_time), (gst_segment_to_running_time),
(gst_segment_clip):
* gst/gstsegment.h:
Added segment helper structure and methods. Not fully implemented
yet.
Added segment check.