In Qt6 QDomDocument does not open the file itself any more

NO_CHANGELOG

Now it creates C++ code that uses QString::arg(Args...) instead of
arg().arg().

AFAICS, the type of the "Args" is QString, so this should work.

- Use more range-for loops where appropriate
- Use auto instead of the usually-long iterator type names
- Use cbegin/cend(), to match the std:: containers, less confusion
- Use qDeleteAll instead of a for loop
- Make a QRE with a long-ish pattern static

NO_CHANGELOG

- Use QString::arg(Args...) instead of .arg().arg()
- Use QLatin1String for string comparisons, should be faster
- Use QLatin1String::arg() for better readability

- Add the comment dfaure suggested in the MR, to explain why it's '2%'
  then '%1' in a QString().arg().arg()

NO_CHANGELOG

This was missed in https://invent.kde.org/frameworks/kconfig/-/merge_requests/41

The implicit conversion fails in Qt6

NO_CHANGELOG

- This has the same logic as QMap::constFind(); less detaching and now we
can use auto keyword when creating iterators and always get a const_iterator
even when calling constFindEntry() on a non-const map.

- Use QCOMPARE() where appropriate in the unit tests.