Added gst_pipeline_add_sink/src to allow multiple sink cases and more complex autoplu...

Added gst_pipeline_add_sink/src to allow multiple sink cases and more complex autoplugging. Update docs too.

Original commit message from CVS:
Added gst_pipeline_add_sink/src to allow multiple sink cases and
more complex autoplugging. Update docs too.
Simplified the pipeline autoplugging code.
Changed the cothread case: One iteration is now a push from the src
element. The disk source does not change its state anymore on eof.
Better type setting for the ac3 parser/decoder and mpeg2parse.