Disallow scrolling beyond end of file.

[chrisvest]

Both keyboard navigation and the "Go to" dialog allow navigating to locations far beyond the end of the file. This needs to be prevented.

• [x] Disallow navigating beyond bottom edge with keyboard movements.
• [x] Jumping to line or offset beyond bottom edge with the "Go to" dialog, must land the viewport at the bottom edge.
• [x] Changing the line width must also move the viewport to the same file offset it was placed at before the line width change.