Highlight tool for bases

Add the ability to highlight one or more bases so that I can track them between natural and target views. This would be useful on large problems where the bases can move around dramatically.

On Mac, you can option-click a nucleotide to highlight it. I think there’s a similar key combination on Windows, but I can’t remember what it is… control? alt?

OK, didn’t know about that. On the Mac, it’s command-click. The black circle is a bit hard to see, so I would still suggest giving it a nice glow instead.