Don't call setlocale("") on startup by default any longer.

Don't call setlocale("") on startup by default any longer.

This undoes the changes of r44773 because calling setlocale() resulted in C
locale being set differently from C++ locale which was confusing and led to
huge slowdowns in any code using std::stream with at least MinGW. And setting
the C++ locale to be the same, as r72719 tried to do, doesn't seem to be
practical as it results in immediate crashes under OS X and MinGW when used
under XP.

Do provide wxApp::SetCLocale() helper to explicitly do what was previously
done implicitly, even though currently it is a trivial wrapper for setlocale()
and we don't even need to call gtk_set_locale() as it has never done anything
else and is deprecated since GTK+ 2.24.

Closes #14780.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72951 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775