gst/playback/: Move setup_output_pads into a virtual function, remove group-switch (no longer needed) and redirect (h...

Original commit message from CVS:
* gst/playback/gstdecodebin.c: (gst_decode_bin_class_init),
(try_to_link_1):
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_class_init),
(group_commit), (probe_triggered), (setup_source),
(gst_play_base_bin_change_state):
* gst/playback/gstplaybasebin.h:
* gst/playback/gstplaybin.c: (gst_play_bin_class_init),
(gst_play_bin_init), (remove_sinks), (setup_sinks),
(gst_play_bin_change_state):
Move setup_output_pads into a virtual function, remove
group-switch (no longer needed) and redirect (handled by bus
now) signals.