add sensitive scrollbar feature

[HelloWorld-er]

Originally, scrollbar in scrollable frame is displayed regardless whether it is necessary. If the content size is smaller than the frame size, the scrollbar of that scrollable frame is not necessary to be displayed. I added a few lines of code to check whether it is necessary to display scrollbar in that frame as <Configure> event is happened.

[HelloWorld-er]

Because I have accidentally deleted the file, I reset it. And also, because of previous misunderstanding, the scrollbar will not be sensitive and the frame may also collapse if the size of the frame is set under a certain value. Now I have updated. So if you want to see the difference, go straight to the last update which should be under this comment.