More non-Qt5-functionality-affecting Qt6 compatibility work

Mostly add QStringLiterals & QLatin1Strings & QLatin1Chars, also some remaining foreaches to fors, add and remove includes, consts & Qt version check for the remaining QNetworkConfigurationManager uses and some changed overload function parameters.

I'll use these changes on Qt5/KF5 a day or two and read through the changed lines and merge. Should be all simple & straightforward, but there's a risk of typos etc.