qt: correct behavioral change with QDateTime and use QStringLiteral more

It appears that I have previously misinterpreted the docs on how QDateTime is constructed when time spec is not provided. This should not be a problem because the information is not lost, but I did not intend to change the behavior there. So I now use QTimeZone::systemTimeZone() for local time zone, which is how it appears to behave in Qt 5.

Additionally, it seems that Qt 6 did not bring the new string view stuff for QRegularExpression constructor, for that, I added QStringLiteral macro in order to not wait on QString construction.

Request review @chub.