Scrollbar

[lilkeet]

The scrollbar is barely functional. Here are some known bugs:

• Clicking and dragging anywhere within a buffer moves the scrollbar and scrolls the buffer.
• The scrollbar gets its size wrong and hits the bottom early, before the end of viewable lines in the document are reached.
• Movement does not keep up with the mouse, so multiple drags are necessary to get anywhere fast.
• Downward movement is slower than upward.
• Activity is not shown when the bar is being dragged unless the mouse is directly on top of it.

This pr fixes the above issues and cleans up this module by refactoring and adding comments.

This pr is based on #27 .