implemented Exit button

Window.py

@@ -41,6 +41,7 @@ class Window:
         in_button = gtk.Button('In')
         out_button = gtk.Button('Out')
         exit_button = gtk.Button('Exit')
+        exit_button.connect("clicked", self.destroy_event)
 
         hbox = gtk.HBox()
         hbox.pack_start(self.mark_label, expand = True, fill = False)
@@ -349,6 +350,7 @@ class Window:
         return False
 
     def destroy_event(self, widget, data=None):
+        # could be called by window or exit button
         gtk.main_quit()
 
     def window_key_press_event(self, widget, event, data=None):