A document is first loaded as a generic PageBase before being converted to
level one pages afterwards.
This is preparation for loading pages of other types such as DRCS and Public
Object Pages. The latter will need further work as the non-TTI file loaders
blindly assume X/1 to X/25 are 7-bit odd parity coded.
This also reverts 0a1c018 putting back the copy constructor that allows
a PageBase to be converted to a LevelOnePage.
"Metadata" is data which is stored in a teletext file format but is not part
of the page itself, such as DE description and CT cycle time in the TTI file
format.
