Update cached values in wxNumberFormatter when locale changes.

Caching the decimal and thousands separators in wxNumberFormatter is a useful
performance optimization, however it can give wrong results if the locale
changed since the cached values were initialized. So remember the locale used
for the initialization and redo it if it changed. This should still be almost
as fast as the previous version but now also correct (still not MT-safe
though).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66713 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775