encoding-profile: Allow using factory names in serialization format
Instead of enforcing the user to know and understand caps to describe
the encoding format, let him use element factory names directly.
This also makes it possible to ensure that a specific encodore/muxer
is used instead of letting the ranking system do it.
It is now possible to describe an encoding format simply specifying:
matroskamux:x264enc:vobisenc
Factor out functions in the parsing, cleaning up the whole thing.
Update documentation.