docs/design/part-overview.txt: Make upsteam/downstream concepts more clear.

docs/design/part-overview.txt: Make upsteam/downstream concepts more clear.

Original commit message from CVS:
* docs/design/part-overview.txt:
Make upsteam/downstream concepts more clear.
Give an example of serialized/non-serialized events.
* docs/design/part-events.txt:
* docs/design/part-streams.txt:
Mention applied_rate.
* docs/design/part-trickmodes.txt:
Mention applied rate, flesh out some more use cases.
* gst/gstevent.c: (gst_event_new_new_segment),
(gst_event_parse_new_segment), (gst_event_new_new_segment_full),
(gst_event_parse_new_segment_full), (gst_event_new_tag),
(gst_event_parse_tag), (gst_event_new_buffer_size),
(gst_event_parse_buffer_size), (gst_event_new_qos),
(gst_event_parse_qos), (gst_event_parse_seek),
(gst_event_new_navigation):
* gst/gstevent.h:
Add applied_rate field to NEWSEGMENT event.
API: gst_event_new_new_segment_full()
API: gst_event_parse_new_segment_full()
* gst/gstsegment.c: (gst_segment_init), (gst_segment_set_seek),
(gst_segment_set_newsegment), (gst_segment_set_newsegment_full),
(gst_segment_to_stream_time), (gst_segment_to_running_time):
* gst/gstsegment.h:
Add applied_rate to GstSegment structure.
Make calculation of stream_time and running_time more correct
wrt rate/applied_rate.
Add some more docs.
API: GstSegment::applied_rate field
API: gst_segment_set_newsegment_full();
* libs/gst/base/gstbasesink.c: (gst_base_sink_configure_segment),
(gst_base_sink_get_sync_times), (gst_base_sink_get_position):
* libs/gst/base/gstbasetransform.c:
(gst_base_transform_sink_eventfunc),
(gst_base_transform_handle_buffer):
Parse and use applied_rate in the GstSegment field.
* tests/check/gst/gstevent.c: (GST_START_TEST):
Add check for applied_rate field.
* tests/check/gst/gstsegment.c: (GST_START_TEST),
(gstsegments_suite):
Add more checks for various GstSegment operations.