API: gst_base_src_query_latency(). Added method so that subclasses can easily get the latency values of the base sour...

Original commit message from CVS:
* docs/libs/gstreamer-libs-sections.txt:
* libs/gst/base/gstbasesrc.c: (gst_base_src_query_latency),
(gst_base_src_default_query), (gst_base_src_get_range):
* libs/gst/base/gstbasesrc.h:
API: gst_base_src_query_latency(). Added method so that subclasses can
easily get the latency values of the base source class.