Merge branch 'killqt4'

Remove all trace of Qt4 support. The different elements have been
considered carefully before removal.

The only changes that need to be done concern Windows and macOS
compilation instruction and will need to be taken care of by the
respective maintainers before prerelease.

The file TODO.killqt4 lists what remains to be done.

diff --cc lib/RELEASE-NOTES
Simple merge

diff --cc lib/symbols
Simple merge

diff --cc src/frontends/qt/GuiFontMetrics.cpp
Simple merge

diff --cc src/frontends/qt/GuiView.cpp
Simple merge