Inkscape 1.2+ saves a multi-page document as <inkscape:page> elements
in its <sodipodi:namedview>; VectorCraft opened it with one artboard
the size of the root, leaving the other pages' art outside any. Each
page with a size now becomes an artboard at its place (through the
viewBox, as the art is), named by its inkscape:label or "Artboard N".
Files without pages keep their one artboard.
Fixes#1601