ext/musepack/: Fix seeking in musepack files (it's still incredibly slow, but I don't think that is our plugin's faul...

Original commit message from CVS:
* ext/musepack/gstmusepackdec.c: (gst_musepackdec_base_init),
(gst_musepackdec_class_init), (gst_musepackdec_init),
(gst_musepackdec_send_newsegment),
(gst_musepackdec_handle_seek_event), (gst_musepackdec_src_event),
(gst_musepackdec_src_query), (gst_musepackdec_src_convert),
(gst_musepack_stream_init), (gst_musepackdec_sink_activate_pull),
(gst_musepackdec_loop), (gst_musepackdec_change_state):
* ext/musepack/gstmusepackdec.h:
* ext/musepack/gstmusepackreader.c: (gst_musepack_reader_peek),
(gst_musepack_reader_seek), (gst_musepack_reader_tell),
(gst_musepack_reader_get_size):
* ext/musepack/gstmusepackreader.h:
Fix seeking in musepack files (it's still incredibly slow, but I
don't think that is our plugin's fault). Clean up code and get
rid of old cruft. Post tags with all kind of neat information like
replay gain and such on the bus, if it is available. Add a
'musepackdec' debug category.