gst/qtdemux/qtdemux.*: Handle stss boxes so we can mark and find keyframes.

gst/qtdemux/qtdemux.*: Handle stss boxes so we can mark and find keyframes.

Original commit message from CVS:
* gst/qtdemux/qtdemux.c: (gst_qtdemux_init),
(gst_qtdemux_push_event), (gst_qtdemux_go_back),
(gst_qtdemux_perform_seek), (gst_qtdemux_do_seek),
(gst_qtdemux_handle_src_event), (plugin_init),
(gst_qtdemux_change_state), (gst_qtdemux_loop_state_movie),
(gst_qtdemux_loop), (gst_qtdemux_chain),
(qtdemux_sink_activate_pull), (gst_qtdemux_add_stream),
(qtdemux_parse), (qtdemux_parse_tree), (qtdemux_parse_trak),
(qtdemux_parse_udta), (qtdemux_tag_add_str), (qtdemux_tag_add_num),
(qtdemux_tag_add_gnre), (gst_qtdemux_handle_esds):
* gst/qtdemux/qtdemux.h:
Handle stss boxes so we can mark and find keyframes.
Implement correct accurate and keyframe seeking.
Use _DEBUG_OBJECT when possible.