Qt >= 5 refuses to display them. We work around this in lib/symbols
already, but make sure we do not try to display math glyphs at these
code points anyway.
Fixes bug #8493.
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.
