gst/playback/: Adding stream selection support plus required properties for applications to use this. Fully fixes #10...

Original commit message from CVS:
* gst/playback/Makefile.am:
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_class_init),
(group_destroy), (group_commit), (group_is_muted),
(gen_preroll_element), (add_stream), (unknown_type),
(probe_triggered), (preroll_unlinked), (mute_stream),
(silence_stream), (new_decoded_pad), (setup_substreams),
(setup_source), (get_active_source), (mute_group_type),
(muted_group_change_state), (set_active_source),
(gst_play_base_bin_set_property), (gst_play_base_bin_get_property),
(play_base_eos), (gst_play_base_bin_change_state):
* gst/playback/gstplaybasebin.h:
* gst/playback/gstplaybin.c: (add_sink), (setup_sinks):
* gst/playback/gststreaminfo.c: (gst_stream_info_class_init),
(gst_stream_info_dispose), (stream_info_mute_pad),
(stream_info_change_state), (gst_stream_info_set_mute):
* gst/playback/gststreamselector.c: (gst_stream_selector_get_type),
(gst_stream_selector_base_init), (gst_stream_selector_class_init),
(gst_stream_selector_init), (gst_stream_selector_dispose),
(gst_stream_selector_get_linked_pad),
(gst_stream_selector_get_caps), (gst_stream_selector_link),
(gst_stream_selector_get_linked_pads),
(gst_stream_selector_request_new_pad), (gst_stream_selector_chain):
* gst/playback/gststreamselector.h:
Adding stream selection support plus required properties for
applications to use this. Fully fixes #100931.