Qt6 port

[marc-sturm]

Qt5 no longer has support.
We need to update

literature:

• https://spyro-soft.com/developers/qt5-to-qt-6migration-step-by-step

• https://www.qt.io/blog/references-and-hints-about-porting-from-qt5-to-qt6

• Adapt the code from Qt 5.12 to Qt 5.15

• Check if Qt 6 offers a better way to build plots (instead of using Python we could do it with C++)

• Switch from Qt 5.15 to Qt 6

It was assumed that XMLPatterns module (deprecated in Qt 6) will be later reintroduced.
In version 6.8 the module is still missing:
https://doc.qt.io/qt-6/qtmodules.html

Only XML module is there

[ubuntolog]

I am working on this branch at the moment qt6_switch

[ubuntolog]

The branch compiles with Qt 5.15 and does not show any warnings now. Unfortunately, I could not compile it with Qt 6. Entire modules have been removed: e.g. xmlpatterns

[ubuntolog]

The new branch cannot be compiled with Qt 5.12: I am getting an error "endl is not a member of Qt"

[ubuntolog]

The master branch now includes the code compatible with qt 5.15 and when compiling with qt 5.15 no warnings will be displayed. For the switch to Qt 6 the following things have to be considered:

• Removal of xmlpatterns module and its replacement with a third-party library (possibly libxml2)
• The remaining expressions using QRegExp need to be replaced with QRegularExpression, exactMatch method was removed, use match and hasMatch with some modifitions
• QT_CHARTS_USE_NAMESPACE replaced by a regular import QtCharts/QChartView
• QGridLayout::setMargin replaced by setContentsMargins
• QTextStream::setCodec("UTF-8") replaced by QTextCodec::codecForName("UTF-8");
• count() and size() method return new types, which causes problems in std::max statements, static_cast is not a good option
• QSvgRenderer replaced by QtSvg/QSvgRenderer and QPainter, adjust and test SVG rendering
• QT += network is needed for almost every project file

Without the above mentioned changes the code cannot be compiled with Qt 6. There will be around 300 warnings after that, and they have to be addressed.

[ubuntolog]

Post on qsizetype: https://forum.qt.io/topic/139340/what-exactly-is-qsizetype-and-what-is-it-s-purpose