Fixes #155. Adds buttons as well as keyboard shortcuts (Ctrl+Up and Ctrl+Down) for scrolling to the previous/next difference. This saves time having to scroll through the whole function to find one tiny mismatch. Supports the function diff view and data diff view.

Note that it specifically looks for the next/prev diff that isn't already visible on your screen. So if all the diffs are onscreen already, the buttons won't scroll between those visible diffs, they'll just do nothing in that case.