Add wxRegEx::QuoteMeta() helper

Quote all characters special for wxRegEx in the given string: this can
be useful when searching for a literal string using wxRegEx.

Closes https://github.com/wxWidgets/wxWidgets/pull/1489

include/wx/regex.h

@@ -142,6 +142,8 @@ public:
     int ReplaceAll(wxString *text, const wxString& replacement) const
         { return Replace(text, replacement, 0); }
 
+    static wxString QuoteMeta(const wxString& str);
+
     // dtor not virtual, don't derive from this class
     ~wxRegEx();