Add wxRegEx::ConvertFromBasic() helper

This will be used to implement support for BREs using PCRE which doesn't
support them directly in the upcoming commits.

include/wx/regex.h

@@ -144,6 +144,9 @@ public:
 
     static wxString QuoteMeta(const wxString& str);
 
+    // return the extended RE corresponding to the given basic RE
+    static wxString ConvertFromBasic(const wxString& bre);
+
     // dtor not virtual, don't derive from this class
     ~wxRegEx();