mkv: Added EbmlTypeDispatcher

The EbmlTypeDispatcher is a type used to "dispatch" an object of type
EbmlElement* to an appropriate callback depending on the dynamic type of
the referred to object.

It can effectivelly replace the massive if-else branches found
throughout the module by not only making the code easier to understand
and maintain, but also by making it _a lot_ faster (benchmarks show a
speed increase between 450 and 700% in terms of lookup).