Although the document class keeps the page number, the sub pages may
need to know the page number as well. For example X/27 packets
contain relative magazine numbers which need the current page number
to calculate the actual page numbers referenced.
Any unhandled packets within pages are now loaded and saved intact
albeit with no way to edit them.
File handling is now two separate processes: parsing TTI files into
packets, and then interpreting the packets into how pages are
displayed. This will allow us to import other file formats, and to deal
with page functions other than Level 1 Teletext pages.
