The setting was imported and editable but never read: the composer, the
canvas grid and baseline snapping measured the grid from the page top.
BaselineGrid::origin gives the first line on a page (start, plus the
page's top margin for Top Margin), and all three use it. The composition
cache tracks the setting and the pages' top margins.
Also adds a left-to-right facing-pages guard test for mirrored margins
and the right parent page's guides and running head.