qt: make use of `QStringBuilder` in `ControlbarProfileModel::save()`

This was unfortunately forgotten in 04507065.

I used % instead of + even though defining QT_USE_QSTRINGBUILDER (where we do) makes + behave like %, to explicitly request to use QStringBuilder noting the amount of additions done.

And switch to constexpr from preprocessor definitions.

Request review @chub.