gst/playback/gstplaybasebin.*: Multiple .sub files is just a stupid idea... Fix some threading mistakes. Interestingl...

Original commit message from CVS:
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_init),
(gen_preroll_element), (remove_groups), (setup_subtitle),
(gen_source_element), (setup_source):
* gst/playback/gstplaybasebin.h:
Multiple .sub files is just a stupid idea... Fix some threading
mistakes. Interestingly, external .sub files cause playbin to
hang, I don't know why... Parsing fixes contributed by Felix
Kooman <fkooman@tuxed.net>.