Refactor precision- and percentage-handling

The precision -1 was used in places where it meant "use the default
precision". For KLocale, this went through decimalPlaces() and
usually returned 2. For QLocale, this is invalid, and 6 is normally used.

This meant that 1% was displayed as 1.000000%, which is kind of useless,
just like 24KiB/s was displayed as 24.000000KiB/s -- implying a
network which could conceivably also transmit 1 bit in a little over
2 minutes, and measure that accurately.

Provide convenience methods (static, inline) for providing more
sensible precisions.