[MusicXML import] allow numbers greater than MAX_NUMBER_LEVEL

Currently the MusicXML importer strictly complies with the specification, which states that the number parameter must be between 1 and 6. See: http://usermanuals.musicxml.com/MusicXML/MusicXML.htm#ST-MusicXML-numbe…. Larger numbers may be found in MusicXML files but should lead to validation errors on import, followed by ignoring the elements whose numbers are too high. If MuseScore allows it (don't know, would have to check), the limits for all stacks could be set higher. In that case the validation error would remain, but the import would be as expected.