#310 Ctrl-z (to center) doesn\'t exactly center

[Brian O.]

Ctrl-z (to center) doesn't exactly center--if you're zoomed in all the
way you'll see it's a bit off. This was reported by a user:

From: Katy Williams <kewilliams@ucdavis.edu>
Also, the "center screen" and "create exon" features are slightly off. To center on what I want, I have to right-click about an inch (on my 12.1 inch screen) to the right of where I want to center.