Add some Right-To-Left language workarounds.

MixerBoard, LyricsWindow, Screenshot and most effects are now LTR in RTL locales.
Also added a pref so this can be overridden.  As RTL affects window creation, the language choice and this preference may only fully take effect after a restart of Audacity.

src/Screenshot.cpp

@@ -308,6 +308,7 @@ void ScreenFrame::Populate()
 void ScreenFrame::PopulateOrExchange(ShuttleGui & S)
 {
    wxPanel *p = S.StartPanel();
+   RTL_WORKAROUND(p);
    {
       S.SetBorder(3);