Support CLDR keyboard mapping files

The Unicode Consortium's Common Locale Data Repository (CLDR) includes the definition of an interchange format for the description of keyboard layouts. This could potentially provide a general framework for extending Qt Virtual Keyboard's support for diverse locales, providing a platform-independent mechanism for adding support for locales.

Things to investigate:

• How widely is the CLDR format used ?

We already digest and incorporate significant amounts of data from CLDR, for use by QLocale and the L10n support in QTimeZone for naming zones. We thus have existing scripts, in qtbase/util/locale_database/, for ingesting data from CLDR, which should ease the incorporation of any CLDR data we may want to include for Qt Virtual Keyboard. The CLDR keyboard format stresses that it is intended as an interchange format, so we would definitely want to digest any data we take from it into a form that can be efficiently accessed at run-time (rather than the raw XML).