gst/playback/gstplaybin.c: Detect NO_PREROLL state change returns and disable clock...

gst/playback/gstplaybin.c: Detect NO_PREROLL state change returns and disable clock distribution to the sinks so that...

Original commit message from CVS:
* gst/playback/gstplaybin.c: (gst_play_bin_class_init),
(gst_play_bin_vis_blocked), (gst_play_bin_set_property),
(gen_video_element), (gen_text_element), (gen_audio_element),
(gen_vis_element), (remove_sinks), (add_sink), (setup_sinks),
(gst_play_bin_set_clock_func), (gst_play_bin_change_state):
Detect NO_PREROLL state change returns and disable clock distribution to
the sinks so that sync is disabled.
Avoid some type checking and do simple casts instead.
Small cleanups, fix some FIXMEs.
Be more robust when linking user specified elements, catch an report
errors. Fixes #357404.
Fix some leaks in the error paths.