demux/mkv: use a better name for unused elements

Unless it's an EbmlDummy the element is not unknown if it has a known class.

Rather than displaying a decorated C++ class name, show the actual element name in libmatroska. No need for Runtime Type Information (RTTI) for that.

The EBML_NAME() has been around since at least libebml 0.8: https://github.com/Matroska-Org/libebml/commit/cd378f23ff5c8bd477fe066fccca532bf00f974b