qt: do not use double precision in settings
The docs state that:
Note that INI files lose the distinction between numeric data and the strings used to encode them, so values written as numbers shall be read back as QString. The numeric value can be recovered using QString::toInt(), QString::toDouble() and related functions.
Currently, double-precision numbers can not be reconstructed properly due to QString's way of handling dp numbers.
Since they are read back as QString, we should save these numbers according to the precision that QString is comfortable with, so that they are parsed properly when the application starts.
Request review @chub.