allow const visitors and use them

alib2/src/alphabet/BlankSymbol.h

alib2/src/alphabet/LabeledSymbol.h

alib2/src/alphabet/Symbol.h

alib2/src/alphabet/SymbolBase.h

alib2/src/alphabet/SymbolToStringComposer.h

alib2/src/alphabet/SymbolToXMLComposer.cpp

alib2/src/alphabet/SymbolToXMLComposer.h

alib2/src/automaton/Automaton.h

alib2/src/automaton/AutomatonBase.h

alib2/src/automaton/AutomatonToXMLComposer.cpp

alib2/src/automaton/AutomatonToXMLComposer.h

alib2/src/automaton/FSM/CompactNFA.cpp

alib2/src/automaton/FSM/CompactNFA.h

alib2/src/automaton/FSM/DFA.h

alib2/src/automaton/FSM/EpsilonNFA.h

alib2/src/automaton/FSM/ExtendedNFA.cpp

alib2/src/automaton/FSM/ExtendedNFA.h

alib2/src/automaton/FSM/NFA.h

alib2/src/automaton/PDA/PDA.h

alib2/src/automaton/TM/OneTapeDTM.h