[PATCH] LibCSS: Completion of widows/orphans support

Attached is a patch to complete libCSS' widows and orphans property support. I've also made small modifications to a few of the other internal 'page' property setters (set_page_break_before) etc. to not set up a page struct if there's not one already and the property's being set to its default value. Especially with non-inherited properties, this seems to save a surprising amount of runtime when using libCSS heavily (informal testing, paginating an ebook), and presumably a fair bit of RAM too, especially as the page struct grows in size. Jamie.