qt: use QT_USE_QSTRINGBUILDER
Qt can optimize string concatenation where operator "+" is used. This is explained in detail here: https://doc.qt.io/qt-6/qstring.html#more-efficient-string-construction.
In order to make use of the string builder, QT_USE_QSTRINGBUILDER
needs to be defined explicitly because this needs additional changes in the code. In our case, there are only two occurrences and I tried to correct them here.
Request review @chub.