gst/playback/: Add mute property.

gst/playback/: Add mute property.

Original commit message from CVS:
* gst/playback/gstplaybin2.c: (gst_play_bin_class_init),
(gst_play_bin_set_property), (gst_play_bin_get_property),
(pad_added_cb), (pad_removed_cb), (no_more_pads_cb):
* gst/playback/gstplaysink.c: (gst_play_sink_set_mute),
(gst_play_sink_get_mute), (gen_audio_chain):
* gst/playback/gstplaysink.h:
Add mute property.
* gst/playback/gststreamselector.c: (gst_selector_pad_event),
(gst_selector_pad_chain):
* gst/playback/gststreamselector.h:
Make sure we forward the event only once.
* tests/examples/seek/seek.c: (stop_cb), (mute_toggle_cb), (main):
Add and implement the mute button for playbin2.