Make _() and friends safe to call from any thread.
The GetUntranslatedString() hack keeps a global copy of all strings, so
that it can return a const reference as wxGetTranslation() return value.
A global wxHashSet instance shared by all threads won't do, even guarded
with a critical section, because it may internally copy values on any
insert and thus invalidate pointers that may still be used on another
thread.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74833
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