Increase slope gradient on back buttons

I don't know why, but while looking at the back buttons I couldn't help but feel that they were slightly too pointy. Feel free to disagree with me, but I think that this pull request makes them slightly more pleasing to look at.

I've calculated the point as a ratio of the button height: it's now 10.5 points for portrait, and 8.5 points for landscape, rather than 13.5 points regardless. I've also added a title position offset to shift the titles left a couple of points on back buttons.