add pagebreakstyles plain and sidebyside to LaTeX output

Here are two new pagebreakstyles that I found useful: "plain" is for page labels that already are sufficiently marked up (for example, already have [] around them).  "sidebyside" is the same but also expects a facs attribute with an image of the page.  The result here is similar to a critical edition, with the image on the even (left) pages, and the page label + following content on the odd (right) pages.

Now, ideally, placing the @facs images and styling the page label would be separate templates.  But that's more than I needed, so here is a first step towards fancier pagebreakstyles.

I guess another solution for me would be to overwrite the whole pb template in the profile.  But a "critical edition" layout seems generally useful, so maybe you like to include it.