Add QDateTimeFormat based on CLDR's format strings

CLDR has a significantly richer language for date-time formatting than ours.
(It would appear the macOS native date-time formatting has similar, possibly based on CLDR. See qlocale_mac.mm for details.)
Rather than lossily converting CLDR's date-time formats to ours when generating QLocale's data, preserve CLDR's date-time formats faithfully and support them correctly.

Encapsulate support for these formats in a QDateTimeFormat class that can be used both for serialization and parsing; it can be the home of a new and better parser for date-times (that matches up the text to the format before it starts trying to determine the values of the fields within it; which should let it read fields in order of decreasing significance instead of from left to right, which should significantly reduce the complexity compared to QDateTimeParser).

We can (possibly lossily, but probably not) convert our legacy date-time formats to CLDR's to eventually replace our date-time parsing and formatting code. The QDateTimeEdit class shall also need a major overhaul in the process.

In particular, note that CLDR provides <dateTimeFormat> elements that say how to combine the date and time formats, which we just naively stick together with a plain space.