gst/base/gstbasesink.*: Store and use discont values when syncing buffers as described in design docs.

Original commit message from CVS:
* gst/base/gstbasesink.c: (gst_base_sink_handle_object),
(gst_base_sink_event), (gst_base_sink_get_times),
(gst_base_sink_do_sync), (gst_base_sink_change_state):
* gst/base/gstbasesink.h:
Store and use discont values when syncing buffers as described
in design docs.

* gst/base/gstbasesrc.c: (gst_base_src_send_discont),
(gst_base_src_do_seek), (gst_base_src_loop), (gst_base_src_start),
(gst_base_src_activate_push):
Push discont event when starting.

* gst/elements/gstidentity.c: (gst_identity_transform):
Small cleanups.

* gst/gstbin.c: (gst_bin_change_state):
Small cleanups in base_time  distribution.

* gst/gstelement.c: (gst_element_set_base_time),
(gst_element_get_base_time), (gst_element_change_state):
* gst/gstelement.h:
Added methods for the base_time of the element.
Some MT fixes.

* gst/gstpipeline.c: (gst_pipeline_send_event),
(gst_pipeline_change_state), (gst_pipeline_set_new_stream_time),
(gst_pipeline_get_last_stream_time):
* gst/gstpipeline.h:
MT fixes.
Handle seeking as described in design doc, remove stream_time
hack.
Cleanups clock and stream_time selection code. Added accessors
for the stream_time.