API: Add gst_uri_protocol_is_supported(), which checks if an sink or src that supports a given URI protocol exists.

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gsturi.c: (get_element_factories_from_uri_protocol),
(gst_uri_protocol_is_supported), (gst_element_make_from_uri):
* gst/gsturi.h:
API: Add gst_uri_protocol_is_supported(), which checks if an sink
or src that supports a given URI protocol exists.