Support dragging and dropping of files onto app icon

Tested on macos and windows.  Drag and drop files onto either the dock icon or task bar icon once SavvyCCAN is running

mainwindow.h

@@ -54,6 +54,8 @@ public:
     CANFrameModel * getCANFrameModel();
     ~MainWindow();
 
+    void handleDroppedFile(const QString &filename);
+
 private slots:
     void handleLoadFile();
     void handleSaveFile();