drop support for storing types that are not QString-convertable via QSettings

drop support for storing types that are not QString-convertable via QSettings

I am not interested in custom types feature at all - QMainWindow for
example saves its state via QDataStream. ofcourse it involves conversion
but that is the case with QVariant aswell. there is also the qreal type
thing - it may be float, it may double. streaming QRectF has to be done
in such a way to account for both cases but here is the catch - if
conversion fails there has to be fallback which QSettings::value() kinda
covers for types that are not composed of several more types.

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>