In QLocale's number-to-string functions, support precision and width options

QString::arg()'s overloads for integral types have fieldWidth, base and fillChar parameters, enabling callers to control how the number is presented. QLocale::toString()'s matching overloads don't support such control, even though the QLocaleData methods it uses to do the job do support them (since they're how QString at least used to do its version). So we have the ability to do formatted localised numbers, but we don't expose it to users (except via QString::arg()'s %L formats, which are limited to the default or system locale).

We should extend QLocale::toString() overloads to support these parameters.