Sync libmatroska with upstream
Reverting "Don't render KaxTimecodeScaleDenominator for the time being"

This reverts commit 2e9201d3bcb213e4d0462e4814e78271e554c6eb.
Reverting "Output KaxTimecodeScaleDenominator if present"

This reverts commit a0b994d52910ee38b8dd8f534ab055d2d4ac99c5.
Add helper function for getting stringified content from mm_mem_io_c
Do not write EBML void elements to XML files

Fixes #801.
Skip <EBMLVoid> elements in XML files

Fixes #802.
Convert mtx::xml::exception based exceptions to mtx::chapter_parser_x ones
Handle 'throw_on_error' flags correctly

See #802: mmg was exiting due to the wrong handling of this flag. Now
it will show error messages for unknown elements.
Cosmetics: indentation
Add unit tests for the ebml_converter_c based classes
Restore old chapter parser error messages
Intentional update due to EBMLVoid-in-XML handling fix
Keep timecodes for PCM tracks

Fixes #804
Refactoring
Add more debugging options for appending
Only append tracks when all required information for the timecode offset is available

Fixes #793.
Refactoring: do not start header guard names with underscores
Update the Italian translation
Update the Italian translation
Add release message, update version number, set version code name