Improvement: Preview window auto scroll option

[vbrdev]

When new content is added to the end of the rst file, then the user has always to scroll the preview to see the new added contents. Even if the preview window is scrolled to the bottom of the document. Maybe an auto scroll option could be added.

Attention: If content ist not added to the end of the rst file and the preview always scrolls to the end of the file, then we loose focus of the edited section. It would be great to "observe" and scroll always to the edited section. Alternatively the auto scroll option could be activated/deactivated with a button in the menu bar of the widget.