subparse: add support for external SSA/ASS subtitles

Submitted by Ilya K

Description

Can't open subtitles in gstreamer (totem mostly) because of this error. There are similar bugs here: https://bugzilla.gnome.org/show_bug.cgi?id=587704 and here: https://bugs.launchpad.net/gst-plugins-base/+bug/402221
Running latest GStreamer and totem on ubuntu maverick
That's the full log of what happens when I run Totem with debug level 2:

(totem:2989): GLib-GObject-WARNING **: value "10752000" of type guint' is invalid or out of range for property connection-speed' of type `guint'
0:00:08.900728007 2989 0x8ac5080 WARN decodebin2 gstdecodebin2.c:1916:type_found: error: Этот файл является текстовым
0:00:08.900758696 2989 0x8ac5080 WARN decodebin2 gstdecodebin2.c:1916:type_found: error: decodebin2 cannot decode plain text files
0:00:08.901170990 2989 0x8ea76e0 WARN basesrc gstbasesrc.c:2550:gst_base_src_loop: error: Внутренняя ошибка передачи данных.
0:00:08.901193324 2989 0x8ea76e0 WARN basesrc gstbasesrc.c:2550:gst_base_src_loop: error: streaming task paused, reason not-linked (-1)
0:00:08.906839287 2989 0x8ef7a10 WARN qtdemux qtdemux_types.c:170:qtdemux_type_get: unknown QuickTime node type iods
0:00:08.906880790 2989 0x8ef7a10 WARN qtdemux qtdemux_types.c:170:qtdemux_type_get: unknown QuickTime node type avc1
0:00:08.906894991 2989 0x8ef7a10 WARN qtdemux qtdemux_types.c:170:qtdemux_type_get: unknown QuickTime node type avcC
0:00:08.906908383 2989 0x8ef7a10 WARN qtdemux qtdemux_types.c:170:qtdemux_type_get: unknown QuickTime node type btrt
0:00:08.906928473 2989 0x8ef7a10 WARN qtdemux qtdemux_types.c:170:qtdemux_type_get: unknown QuickTime node type chpl
0:00:08.909919508 2989 0x8ef7a10 WARN qtdemux qtdemux.c:5810:qtdemux_parse_trak: unknown version 00000000
0:00:08.958753741 2989 0x8ac5080 WARN totem bacon-video-widget-gst-0.10.c:2093:bvw_bus_message_cb: Warning message: warning message from element 'decodebin21': GstMessageWarning, gerror=(GError)NULL, debug=(string)"gstdecodebin2.c(1916):\ type_found\ ():\ /GstPlayBin2:play/GstURIDecodeBin:uridecodebin1/GstDecodeBin2:decodebin21:\012decodebin2\ cannot\ decode\ plain\ text\ files";
0:00:08.981539951 2989 0x8f4f330 WARN pulse pulsesink.c:558:gst_pulsering_stream_underflow_cb: Got underflow
0:00:08.984938230 2989 0x8f4f330 WARN pulse pulsesink.c:558:gst_pulsering_stream_underflow_cb: Got underflow
0:00:08.985001837 2989 0x8f4f330 WARN pulse pulsesink.c:558:gst_pulsering_stream_underflow_cb: Got underflow
0:00:08.985043041 2989 0x8f4f330 WARN pulse pulsesink.c:558:gst_pulsering_stream_underflow_cb: Got underflow
0:00:08.985081469 2989 0x8f4f330 WARN pulse pulsesink.c:558:gst_pulsering_stream_underflow_cb: Got underflow
0:00:09.054364084 2989 0x8ac5080 WARN totem bacon-video-widget-gst-0.10.c:1604:bvw_handle_element_message: Unhandled element message playbin2-stream-changed from play: element message from element 'play': playbin2-stream-changed, uri=(string)".../video.mp4", suburi=(string)".../video.ass";
(cut off the paths as they're too long)