toc: put toc directly into event/message/query structure

toc: put toc directly into event/message/query structure

Now that TOCs are refcounted and have a GType, we can just
stuff a ref of the TOC directly into the various toc
event/message/query structures and get rid of lots of
cracktastic GstStructure <-> GstToc serialisation and
deserialisation code. We lose some TOC sanity checking
in the process, but that should really be done when
it's being created anyway.