gst/gstmessage.c: Use GstQuark for messages.

gst/gstmessage.c: Use GstQuark for messages.

Original commit message from CVS:
* gst/gstmessage.c: (gst_message_new_error),
(gst_message_new_warning), (gst_message_new_info),
(gst_message_new_buffering), (gst_message_new_state_changed),
(gst_message_new_clock_provide), (gst_message_new_clock_lost),
(gst_message_new_new_clock), (gst_message_new_segment_start),
(gst_message_new_segment_done), (gst_message_new_duration),
(gst_message_new_async_start), (gst_message_parse_buffering),
(gst_message_parse_state_changed),
(gst_message_parse_clock_provide), (gst_message_parse_clock_lost),
(gst_message_parse_new_clock), (gst_message_parse_error),
(gst_message_parse_warning), (gst_message_parse_info),
(gst_message_parse_segment_start),
(gst_message_parse_segment_done), (gst_message_parse_duration),
(gst_message_parse_async_start):
Use GstQuark for messages.