Improve file navigation

[JohnSchmeichel]

I got used to left/right arrows to navigate between files so extending that to the files view (which also fixes a crash as those commands assumed a PatchDocument).

Also tweaking the file navigation a bit to make behavior a bit more consistent between the first and last files:

1. If you are in the middle of a file then left arrow brings you to the start of that file: for long files found not having the ability to jump to the top of the file a bit limiting (could navigate to the previous then forward again, but then it was different for the first file)
2. If you are in the last file then right arrow brings you to the last line of the document: found it very frustrating not being able to jump to the last line of the file.
3. Change page up/down behavior by maintaining line position where possible and allowing the last line to be selected.
4. Change hunk navigation behavior by selecting the start or end of the hunk if navigation has reached the first or last hunk, respectively.