Do number-formatting correctly

The placement of grouping characters in the whole-number portion of a number is currently done wrong. For (only) Indian, we special-case a group of the three least-significant digits with successive groups of two to the left of it; for everything else, it's all three-digit groups. The CLDR contains data that properly describes the correct placement of digits, with one size for the least-significant group and another for all more significant groups. We need to capture that data from CLDR for inclusion in our locale data and adapt the code to use it correctly.

This shall involve discarding the Indian-numbering-specific implementation; at the same time, the separator should be renamed from "thousands" separator.
Fortunately, "thousands" isn't used in the QLocale API: QLocale::NumberOption already uses the name "grouping separator", so we can reuse that.
QLocaleData::Flags needs its ThousandsGrouping renamed to GroupDigits.

The decimal separator and grouping "characters" also need to become QString (as they already are in QSystemLocale::query(QSystemLocale::QueryType, QVariant)), since they may be surrogate pairs in some numbering systems – see QTBUG-69324.