allow reading at the top level with EbmlHead+doctype top elements

[robUx4]

To start reading an EBML file, one needs to use FindNextID() allowing only a single top level element to be detected, or with FindNextElement() which can find any element that is inside an EbmlSemanticContext. But it's currently not possible to combine EbmlHead with, for example, KaxSegment. They have their parent semantic hardcoded to nullptr.

In addition a CRC32 global element should never be allowed if it's not in an EbmlMaster (and may even has to be the first child?). Void may also not be allowed there (I need to check the RFC). So global elements should be handled in a special way there.

Maybe we need a special API to get the parsing started with both the EbmlHead (which is known to libebml) and the top level semantic to use.

[robUx4]

libmatroska has KaxMatroska But there's still the problem of global elements handling. Eventually it might be better to just pass a list of (2) elements allowed at that level.