mkclean & mkvalidator releases

We just released some minor updates to our validation and cleaning tools mkvalidator & mkclean. They now support the special case of alt-ref frames in WebM with a duration of 0.

There are also internal changes made to prepare libebml2 and libmatroska2, our new C libraries for a future release. The libraries are now more modular and easier to integrate without the need for coremake.