Change register to match lines when headings are present about pressbooks-book

[greatislander]

This is a nice print layout thing that @hughmcguire was interested in, I think— basically the idea is that if you have two pages facing each other, and a heading appears within the text on one of the pages, the vertical alignment should allow the lines of body text to line up perfectly. Here's an example of this not happening (in Apple Pages):

In the ideal world, PDF/print themes would use line height and margins around headers etc. to make sure that lines of text remained aligned across facing pages. I think that's what this issue was about. Not an easy proposition but would be pretty cool!