gst/playback/: Implement group-switch signal for use in apps to clear metadata cache...

gst/playback/: Implement group-switch signal for use in apps to clear metadata cache, clean up subtitle, add suburi p...

Original commit message from CVS:
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_class_init),
(gst_play_base_bin_init), (gst_play_base_bin_dispose),
(probe_triggered), (new_decoded_pad), (gen_source_element),
(gst_play_base_bin_set_property), (gst_play_base_bin_get_property):
* gst/playback/gstplaybasebin.h:
* gst/playback/gstplaybin.c: (gst_play_bin_class_init),
(gst_play_bin_init), (group_switch), (remove_sinks), (setup_sinks),
(gst_play_bin_change_state):
Implement group-switch signal for use in apps to clear metadata
cache, clean up subtitle, add suburi property instead of # hack,
some error-out fixes.