Adriaan de Groot authored Jun 27, 2020 and Arjen Hiemstra committed Jul 01, 2020

The port from KLocale -> QLocale left out the format-specifier
in the call to QLocale::toString(double, char, int), so we
ended up with the "precision" specifier as the char parameter --
speficying the format char. It looks like 0 (NUL) is interpreted
as 'f' on the Qt side, and precision is then 6.

The result was tooltip output like "31,405,564.000000 KiB"
which is nonsense -- there is no reason for six digits
of precision when displaying KiB until such a time as
micro-kibibytes ( 1/122 of a bit) are addressable.

Force format 'f' to avoid scientific notation, and
set precision to 0 for integer sensors, -1 (I assume that becomes
6 again internally in QLocale::toString()) otherwise.
The static_assert() is there as a reminder of which method
in QLocale should be called here.