ShuttlegGUI: const wxArrayStringEx & arguments, not wxArrayString *...

... for choice, combo, and listbox; reference allows passing temporaries,
eliminating need for some variables to hold the string arrays.

src/export/ExportMP2.cpp

@@ -93,7 +93,7 @@ public:
    bool TransferDataFromWindow() override;
 
 private:
-   wxArrayString mBitRateNames;
+   wxArrayStringEx mBitRateNames;
    std::vector<int> mBitRateLabels;
 };