Teach QLocale's CLDR infrastructure about singular, plural, many, few &c.

QLocale uses CLDR as a source of data. Some of the things for which it uses that (notably the newly-added byte-quantity printing by formattedDataSize) should really attend to the rules for singular, plural, many, few, zero, one, two and so on, that the CLDR also defines.

I've read somewhere that our translation infrastructure supports those rules, so it would make sense to share its infrastructure for this.