Lots of fixes to the book class: better breaking, right running headers in sections.
Break at best point, don’t break before discardables.
Make paragraph separator more configurable.
???
Better debugging.
Use settings not subclassing.
pushBack is always going to be a hack, but this at least fixes #3.
We do need this after all.
Fix parskip/leading, (AGAIN) allow settings to be nil, add/fix some plain commands.
Whitespace fix.