Get rid of Qt4 code in src/

Get rid of Qt4 code in src/

Qt4 code has been identidifed by
  git grep -l 'QT_VERSION.*\(0x050000\|0x04\)' src

Uses of QT_VERSION_CHECK have been checked too.

been used to remember what places need further work.

Then remaining Qt4 references have been searched using
  git grep -i qt4
  git grep -i 'qt 4'
and relevant issues have been either fixed or noted in the new
TODO.killqt4 file.