gst/playback/: Rough port of playbin. Needs some more work, but is mostly done, and uses a few locks in important pla...

Original commit message from CVS:
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_get_type),
(gst_play_base_bin_class_init), (gst_play_base_bin_finalize),
(get_active_group), (get_building_group), (group_destroy),
(group_commit), (check_queue), (queue_overrun),
(queue_threshold_reached), (queue_out_of_data),
(gen_preroll_element), (remove_groups), (unknown_type),
(add_element_stream), (no_more_pads), (probe_triggered),
(preroll_unlinked), (new_decoded_pad), (setup_subtitle),
(setup_substreams), (setup_source), (finish_source),
(prepare_output), (muted_group_change_state),
(gst_play_base_bin_set_property), (gst_play_base_bin_get_property),
(gst_play_base_bin_change_state):
* gst/playback/gstplaybasebin.h:
* gst/playback/gstplaybin.c: (gst_play_bin_class_init),
(gst_play_bin_init), (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_change_state):
* gst/playback/gststreaminfo.c: (gst_stream_info_class_init),
(cb_probe), (gst_stream_info_new), (gst_stream_info_dispose),
(stream_info_change_state), (gst_stream_info_set_mute),
(gst_stream_info_get_property):
* gst/playback/gststreaminfo.h:
* gst/playback/gststreamselector.c: (gst_stream_selector_init),
(gst_stream_selector_get_linked_pad),
(gst_stream_selector_getcaps),
(gst_stream_selector_get_linked_pads),
(gst_stream_selector_request_new_pad), (gst_stream_selector_chain):
* gst/playback/gststreamselector.h:
Rough port of playbin. Needs some more work, but is mostly done,
and uses a few locks in important places, which should make stuff
like chain-switches clean. Still uses GST_STATE() in a few places,
which isn't all that good an idea, subtitles/elements disabled
because no elements to test with and thus probably broken, query
and event handling moved to GstBin, internal thread removed
alltogether because the pipeline does that for us now. Can play
Ogg/Vorbis files. Haven't tested anything else yet.