gst/playback/: More playbin fixes. Added README. Do better element filtering.

gst/playback/: More playbin fixes. Added README. Do better element filtering.

Original commit message from CVS:
* gst/playback/Makefile.am:
* gst/playback/README:
* gst/playback/gstdecodebin.c: (gst_decode_bin_get_type),
(gst_decode_bin_class_init), (gst_decode_bin_factory_filter),
(compare_ranks), (print_feature), (gst_decode_bin_init),
(gst_decode_bin_dispose), (find_compatibles), (close_pad_link),
(try_to_link_1), (new_pad), (close_link), (type_found),
(gst_decode_bin_set_property), (gst_decode_bin_get_property),
(plugin_init):
* gst/playback/gstplaybasebin.c: (gst_play_base_bin_get_type),
(gst_play_base_bin_class_init), (gst_play_base_bin_init),
(gst_play_base_bin_dispose), (rebuild_pipeline), (queue_overrun),
(gen_preroll_element), (no_more_pads), (new_stream),
(setup_source), (gst_play_base_bin_set_property),
(gst_play_base_bin_get_property), (gst_play_base_bin_change_state),
(gst_play_base_bin_add_element),
(gst_play_base_bin_remove_element),
(gst_play_base_bin_mute_stream), (gst_play_base_bin_link_stream),
(gst_play_base_bin_unlink_stream),
(gst_play_base_bin_get_streaminfo):
* gst/playback/gstplaybasebin.h:
* gst/playback/gstplaybin.c: (gst_play_bin_get_type),
(gst_play_bin_class_init), (gst_play_bin_init),
(gst_play_bin_dispose), (gst_play_bin_set_property),
(gst_play_bin_get_property), (gen_video_element),
(gen_audio_element), (setup_sinks), (gst_play_bin_change_state),
(gst_play_bin_get_event_masks), (gst_play_bin_send_event),
(gst_play_bin_get_formats), (gst_play_bin_convert),
(gst_play_bin_get_query_types), (gst_play_bin_query),
(plugin_init):
* gst/playback/gststreaminfo.c: (gst_stream_type_get_type),
(gst_stream_info_get_type), (gst_stream_info_class_init),
(gst_stream_info_init), (gst_stream_info_new),
(gst_stream_info_dispose), (gst_stream_info_set_property),
(gst_stream_info_get_property):
* gst/playback/gststreaminfo.h:
* gst/playback/test.c: (gen_video_element), (gen_audio_element),
(main):
* gst/playback/test2.c: (main):
* gst/playback/test3.c: (update_scale), (main):
More playbin fixes. Added README. Do better element filtering.
Added base class to preroll media. Added test apps.