Fix two types of Clazy warnings

[gruenich]

Chained QString::arg, replace inclusion of QtCore

[firewave]

It appears multi-arg only works when the arguments are QString.

The GUI now reports error to the terminal (see https://trac.cppcheck.net/ticket/12316):

Could not parse stylesheet of object CodeEditor(0x7fffe6f7de60, name = "CodeEditor")

The code in question is in CodeEditor::generateStyleString():

    QString bgcolor = QString("background:rgb(%1,%2,%3);")
                      .arg(mWidgetStyle->widgetBGColor.red(),
                           mWidgetStyle->widgetBGColor.green(),
                           mWidgetStyle->widgetBGColor.blue());

All of the arguments are int and the called function appears to be QString::arg(int a, int fieldWidth, int base, QChar fillChar) according to my IDE. The actual multi-arg function I see in the header though is QString::arg(const QString &a1, const QString &a2, const QString &a3).

This is with Qt5 though. I have not checked if Qt6 has additional overloads.

[firewave]

Qt6 has a varargs overload but that also only works with strings.

From https://doc.qt.io/qt-6/qstring.html#arg-14:

Args can consist of anything that implicitly converts to [QString](https://doc.qt.io/qt-6/qstring.html), [QStringView](https://doc.qt.io/qt-6/qstringview.html) or [QLatin1StringView](https://doc.qt.io/qt-6/qlatin1stringview.html).

In addition, the following types are also supported: [QChar](https://doc.qt.io/qt-6/qchar.html), [QLatin1Char](https://doc.qt.io/qt-6/qlatin1char.html).

[chrchr-github]

It sounds like the clazy check should detect exactly this kind of misuse that we have now:
https://github.com/KDE/clazy/blob/master/docs/checks/README-qstring-arg.md
Edit: from the bottom:
Using these misleading overloads is perfectly valid, so only warning (1) is enabled by default. To enable warning (2), export CLAZY_EXTRA_OPTIONS="qstring-arg-fillChar-overloads
Maybe we should run clazy in our CI?

[firewave]

Maybe we should run clazy in our CI?

I thought about that but it doesn't offer that much of interest and has several false positives.

My IDE supports Clazy but it seems I am not able to specify the extra option.

[chrchr-github]

@gruenich Can you revert the arg() change?

[gruenich]

@gruenich Can you revert the arg() change?

I'd prefer a partial revert to get it running on Ubuntu 20.04, too. Focal provides an older version of Qt 5.12 which might lack some features.
To identify the breaking part of the changes, I compiled the tip of main on a Ubuntu Focal Docker. It just compiled without an error, so I have no indication which parts of my change are good and which are not.
Further, running Clazy with the extra checks (CLAZY_EXTRA_OPTIONS="qstring-arg-fillChar-overloads") did not help neither.

[firewave]

Reverted in #5902.