This now keeps track of everything that is going on, creates
a tree of chains and groups to allow "demuxer after demuxer" scenarios
and allows chained Oggs with multiple streams (needs oggdemux or playbin2 fixes).

Also document everything in detail and give a general overview of what
decodebin2 is doing at the top of the sources.

Fixes bug #596183, #563828 and #591677.