Include QLocale::system() in matchingLocales() return lists

QLocale::matchingLocales() only returns matches from the CLDR database; it does not include the current system locale as a match, even if it does indeed match.
Check for any possible adverse impacts of including it; if there are none, include it.
If there are adverse impacts, consider adding some way to control whether it is to be included or not, if it matches.

Once that's resolved, check other modules for uses of matchingLocales() and amend them to take account of this change; some of them may be, like qttools/src/shared/qtpropertybrowser/qtpropertymanager.cpp, be including the system locale explicitly themselves (so we need to avoid duplicating it); others may be cases where the system locale should be avoided.