use enum classes in XML parsers

alib2/src/ExceptionFromXMLParser.cpp

alib2/src/ExceptionToXMLComposer.cpp

alib2/src/alphabet/SymbolFromXMLParser.cpp

alib2/src/alphabet/SymbolToXMLComposer.cpp

alib2/src/automaton/AutomatonFromXMLParser.cpp

alib2/src/automaton/AutomatonToXMLComposer.cpp

alib2/src/label/LabelFromXMLParser.cpp

alib2/src/label/LabelToXMLComposer.cpp

alib2/src/regexp/RegExpFromXMLParser.cpp

alib2/src/regexp/RegExpToXMLComposer.cpp

alib2/src/sax/SaxComposeInterface.cpp

alib2/src/sax/SaxParseInterface.cpp

alib2/src/sax/Token.h

alib2/src/string/StringFromXMLParser.cpp

alib2/src/string/StringToXMLComposer.cpp