Rework the ESC key to stop playback as an application event handler...

... So it does not depend on the focused window, and can work when selection
toolbar has focus.  But TrackPanel's escape handling may still take precedence
over it.

src/AudacityApp.h

@@ -132,6 +132,8 @@ class AudacityApp final : public wxApp {
 
    void OnReceiveCommand(AppCommandEvent &event);
 
+   void OnKeyDown(wxKeyEvent &event);
+
    void OnTimer(wxTimerEvent & event);
 
    // IPC communication