Revision 197578

Support for PageLists and PageSets.
A PageList is a sequence of pages in the process of
being laid out from within an fo:page-sequence.  The
list is made up of items implementing the PageListElement
interface; Pages and PageSets.
A PageSet is a set of items implementing the
PageSetElement interface; Pages and PageLists.  It
represents a set of layout attempts, rooted at the same
point in the FO tree, and with a common Page predecessor
(if any).  That is, it hold a set of parallel layout attempts,
which may be individual Pages or PageLists.