A first (rude) attempt at autoplug.

A first (rude) attempt at autoplug.

Original commit message from CVS:
A first (rude) attempt at autoplug.
Autoplugging selects appropriate codecs to connect src to sink, adds
them to the pipeline and connect pads.
Autoplugging will run the typedetect plugin if the src pad has no MIME
type.
No autoplugging is done on the src and sink pads, it's hardcoded:
connect 'src to sink'.
No attempt at creating threads.
No attempt at dynamically autoplugging not yet existing pads.
Changes to (some) plugins to properly set their MIME types.