more image window tweaks

- add image windows to View menu
- add setting to replace old image view instead of creating a new one
- add setting to turn on/off box
- add setting to turn on/off axes

tools/lammps-gui/imageviewer.h

@@ -53,9 +53,10 @@ private slots:
     void do_rot_down();
     void change_group(int);
 
+public:
+    void createImage();
 private:
     void createActions();
-    void createImage();
     void updateActions();
     void saveFile(const QString &fileName);
     void scaleImage(double factor);