Further improve caching of locale-specific data in wxNumberFormatter.

Further improve caching of locale-specific data in wxNumberFormatter.

Also update the cached data if setlocale() was called explicitly instead of
using wxLocale to change the locale because at least under Unix systems
calling setlocale() changes the result of wxLocale::GetInfo() and so the
result returned by wxNumberFormatter::GetDecimalSeparator() and
GetThousandsSeparatorIfUsed() could be inconsistent with the locale being
really used.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66726 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775