decodebin2: Rewrite autoplugging and how groups of pads are exposed
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.
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