August 2008
 * clue-numbering
 * Added "instructions" element
 * is-link to clue type
 * solve-status to cell-type

Feb 2011
 * title-left, title-height
 * scroll-width, scroll-height
 * enforce-alphabet

Nov 2011
 * type-skip-existing

May 2013
 * clue-background-color

Oct 2016
 * fix a few small errors

Feb 2021
 * added clue-font-lines
 * added support for diagonal arrows via arrow-from-position-enum
 * added background-shape-color