EPUB writer:  don't use page-progression-direction in EPUB2.

Also, if page-progression-direction not specified in metadata,
don't include the attribute even in EPUB3; not including it is
the same as including it with the value "default", as we did before.

Closes #1550.
Docx reader: Get rid of ListItem ParPart type.

It's just a paragraph with its own style. Making it into a different
type makes other parsing operations more difficult, and muddies up the
docx xml tree.
ListBuilder: started messing around.
Some more.
Fiddle with it a bit more.
Move it to Lists module.
Start next stage.
Fixed listinfo.