Support scrolling viewport vertically without affecting cursor location.

[ta0kira]

It's often useful to scroll to another part of the document to check on something without losing the cursor position. Right now this isn't possible with FixedWidthEditor and FixedWidthViewer.

I think the implementation would be something like this:

• Add a new function to FixedFontViewer for actions to change the view, e.g., move up or down by n lines.
• The getCursor return probably needs to be made Maybe (Int,Int) to disable its display.
• In the implementation of Document:
  • Add a Maybe (Preview c b) field, where data Preview is just a line zipper containing the whole doc.
  • If the preview exists, use that for visible lines, and don't show the cursor.
  • If edits happen or if the cursor is moved (to include width resizing), set the preview to Nothing.

[ta0kira]

This was partially fixed with 1bc9f7f; however, the implementation for EditingDocument doesn't support scrolling the cursor out of view.