Make disappearing scroll bars optional.

The disappearing scrollbars are completely non-obvious. (of course!) Blender should go back to having scroll bars always on by default with a thicker width. This is much better for newbies. The whole point of a scroll bar being visible is to make it obvious when there is more information that is hidden. For people who think they are too intrusive, then allow them to make them invisible and to set whatever thickness they want when the bars do become visible.