OGM text support, Matroska UTF-8 text support, deadlock fixes all over the place, subtitle awareness in decodebin/pla...

Original commit message from CVS:
* configure.ac:
* ext/ogg/gstoggdemux.c: (gst_ogg_pad_new):
* ext/ogg/gstogmparse.c: (gst_ogm_text_parse_get_type),
(gst_ogm_text_parse_base_init), (gst_ogm_text_parse_init),
(gst_ogm_parse_get_sink_querytypes), (gst_ogm_parse_sink_convert),
(gst_ogm_parse_sink_query), (gst_ogm_parse_chain),
(gst_ogm_parse_plugin_init):
* ext/pango/gsttextoverlay.c: (gst_textoverlay_linkedpads),
(gst_textoverlay_link), (gst_textoverlay_getcaps),
(gst_textoverlay_event), (gst_textoverlay_video_chain),
(gst_textoverlay_loop), (gst_textoverlay_init), (plugin_init):
* ext/pango/gsttextoverlay.h:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_add_stream),
(gst_matroska_demux_handle_seek_event),
(gst_matroska_demux_sync_streams),
(gst_matroska_demux_parse_blockgroup),
(gst_matroska_demux_subtitle_caps),
(gst_matroska_demux_plugin_init):
* gst/matroska/matroska-ids.h:
* gst/playback/gstdecodebin.c: (close_pad_link):
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_init),
(gen_preroll_element), (remove_groups), (add_stream),
(new_decoded_pad), (setup_subtitles), (gen_source_element),
(setup_source):
* gst/playback/gstplaybasebin.h:
* gst/playback/gstplaybin.c: (gen_text_element), (setup_sinks):
* gst/subparse/Makefile.am:
* gst/subparse/gstsubparse.c: (gst_subparse_get_type),
(gst_subparse_base_init), (gst_subparse_class_init),
(gst_subparse_init), (gst_subparse_formats),
(gst_subparse_eventmask), (gst_subparse_event),
(gst_subparse_handle_event), (convert_encoding), (get_next_line),
(parse_mdvdsub), (parse_mdvdsub_init), (parse_subrip),
(parse_subrip_deinit), (parse_subrip_init), (parse_mpsub),
(parse_mpsub_deinit), (parse_mpsub_init),
(gst_subparse_buffer_format_autodetect),
(gst_subparse_format_autodetect), (gst_subparse_loop),
(gst_subparse_change_state), (gst_subparse_type_find),
(plugin_init):
* gst/subparse/gstsubparse.h:
* gst/typefind/gsttypefindfunctions.c: (ogmtext_type_find),
(plugin_init):
Add subtitle support, .sub parser (supports SRT and MPsub),
OGM text support, Matroska UTF-8 text support, deadlock fixes
all over the place, subtitle awareness in decodebin/playbin
and some fixes to textoverlay to handle subtitles in a stream
correctly. Fixes #100931.