Migration to Qt6

The following changes were applied to adapt the GUI to Qt6:

• Adapted GitLab-CI script to Qt6.
• Adapted CMake scripts to Qt6
• Updated qcustomplot files based on the version in the OpenHKL repo.
• Replaced some signals and slots which were obsolete in Qt6.
• Replaced QRegExp with QRegularExpression and QRegularExpressionMatch, since QRegExp is deprecated in Qt6.
• Replaced QStringRef with QString, since QStringRef is deprecated in Qt6.
• Corrected the comparison of QStringView with std::string: QStringView must be converted to a string before being compared to a std::string.
• Added proper include statements to avoid incomplete-type error.
• Used explicit type instead of auto, otherwise the compiler could not determine the type on its own.
• Added an auxiliary function QListToQSet to convert a QList to a QSet; the direct conversion is not allowed in Qt6.
• Discarded QMetaType::registerComparators which is removed in Qt6: In Qt 6, QMetaType::registerComparators have been removed. The metatype system will instead know about those automatically. See https://www.qt.io/blog/whats-new-in-qmetatype-qvariant.
• Windows installer: The libraries libEGL.dll and libGLESv2.dll are not part of the Qt6 distribution; instead, Qt6OpenGL.dll, Qt6OpenGLWidgets.dll and opengl32sw.dll must be included in the Windows installer. See further https://doc.qt.io/qt-6/windows-deployment.html and https://doc.qt.io/qt-6/opengl-changes-qt6.html.
• The emission of the modified signal is modified in ProjectDocument::saveProjectFile, since the original mechanism was logically incorrect.

NOTE:

The CMake command to find Qt6 include files and libraries is the following:

OPT_DIR="$HOME/.local/opt"
QT_CMAKE_DIR="$OPT_DIR/Qt/6.2.3/gcc_64/lib/cmake"
cmake -S . -B build -DCMAKE_PREFIX_PATH="$OPT_DIR;$QT_CMAKE_DIR"

See further:

https://doc.qt.io/qt-6/cmake-commands-qtcore.html
https://doc.qt.io/qt-6/qt-add-resources.html

Closes issue #204 (closed)