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.cpp

@@ -721,6 +721,11 @@ void TrackPanel::UpdateStatusMessage( const wxString &st )
    mListener->TP_DisplayStatusMessage(status);
 }
 
+bool TrackPanel::TakesFocus() const
+{
+   return true;
+}
+
 void TrackPanel::UpdateSelectionDisplay()
 {
    // Full refresh since the label area may need to indicate