Add busy field and quark for the buffering query so that the app can only use the query to see if buffering is in pro...

Original commit message from CVS:
* docs/design/part-buffering.txt:
* gst/gstquark.c:
* gst/gstquark.h:
* gst/gstquery.c: (gst_query_parse_latency),
(gst_query_new_buffering), (gst_query_set_buffering_percent),
(gst_query_parse_buffering_percent):
* gst/gstquery.h:
Add busy field and quark for the buffering query so that the app can
only use the query to see if buffering is in progress.