[janezd]

• Fixes Maximize horizontal space #62: increases the width of the text by the width of @ajdapretnar's finger.
• Prose width is now defined as a constant, so we can easily refine it later if we wish.
• If the book contains no chapters, the left margin width is at least the width of the side note. This allows books in which the entire content as intro to have side notes on smaller screens.

Don't merge before #68, otherwise this may cause overlapping side notes in existing books.