Add wxTranslations::GetTranslatedString().

Replace GetString(), which always returns something (possibly the
original string) with GetTranslatedString() that returns either a
pointer to translated string or NULL.

This simplifies the code a bit, all handling of missing translations is
now done in wxGetTranslation().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74836 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775