Do not return copies of string members

Do not return copies of string members

This fixes the g++ 12 warnings below.

../../master/src/Converter.cpp:714:55: warning: possibly dangling reference to a temporary [-Wdangling-reference]
  714 |                                         Mover const & mover = getMover(conv.to());
      |                                                       ^~~~~
../../master/src/Converter.cpp:714:71: note: the temporary was destroyed at the end of the full expression ‘lyx::getMover(lyx::Converter::to() const())’
  714 |                                         Mover const & mover = getMover(conv.to());
      |                                                               ~~~~~~~~^~~~~~~~~~~
../../master/src/Converter.cpp:786:39: warning: possibly dangling reference to a temporary [-Wdangling-reference]
  786 |                         Mover const & mover = getMover(conv.from());
      |                                       ^~~~~
../../master/src/Converter.cpp:786:55: note: the temporary was destroyed at the end of the full expression ‘lyx::getMover(lyx::Converter::from() const())’
  786 |                         Mover const & mover = getMover(conv.from());