Add support for Qt6, keep Qt5, remove Qt4

[ursfassler]

Summary

Add support for Qt6.

Details

Adding support (and using) Qt6. The officially not supported Qt4 is removed. We now support Qt5 and Qt6. Both versions are tested within the CI.

The CMake option CUKE_ENABLE_QT is replaced with either CUKE_ENABLE_QT_6 or CUKE_ENABLE_QT_5 to give better control to the user as both versions can be installed side by side. CMake version 3.16 is now needed instead of 3.9.

The CalcQt example was refactored to be a bit more modern and compatible with Qt6.

Motivation and Context

Qt6 is the current version, Qt5 is still active but Qt4 is no longer supported by the developers. This changes reflect the current state.

How Has This Been Tested?

CI builds, runs unit test and examples for Qt5 and Qt6.
CalcQt example (GUI application) tested manually.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Breaking change should only be the mentioned CMake options and CMake version.

Checklist:

• It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• I've added tests for my code.
• I have verified whether my change requires changes to the documentation
• My change either requires no documentation change or I've updated the documentation accordingly.
• My branch has been rebased to main, keeping only relevant commits.