Add busy field and quark for the buffering query so that the app can only use the...

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.