Prepare string handling for Qt 6

Since 5.85.0, the ECM compiler settings set:

QT_NO_CAST_TO_ASCII
QT_NO_CAST_FROM_ASCII
QT_NO_URL_CAST_FROM_STRING
QT_NO_CAST_FROM_BYTEARRAY
QT_USE_QSTRINGBUILDER

This prevents automatic string conversions (such as QString s = "text").

Use QStringLiteral, QString::fromUtf8(), explicit QUrl constructors etc.