gst/gstelement.*: New function for setting element time taking into account a hardware buffering delay.

Original commit message from CVS:
* gst/gstelement.h:
* gst/gstelement.c (gst_element_set_time_delay): New function for
setting element time taking into account a hardware buffering
delay.
(gst_element_set_time): Now just an invocation of
gst_element_set_time_delay.
* gst/gstclock.h:
* gst/gstclock.c (gst_clock_get_event_time_delay): New function
allowing to set event times in the future.
(gst_clock_get_event_time): Now just an invocation of
gst_clock_get_event_time_delay.