Bug1917: TrackPanel should keep focus after click or drag...

... though ruler won't, and if you start to drag in TrackPanel but abort it
with the ESC key, that too returns focus

src/TrackPanel.h

@@ -482,6 +482,8 @@ protected:
 
    void UpdateStatusMessage( const wxString &status ) override;
 
+   bool TakesFocus() const override;
+   
    // friending GetInfoCommand allow automation to get sizes of the
    // tracks, track control panel and such.
    friend class GetInfoCommand;