Handle page tree node with no /Type

Original reported here:
https://bugs.launchpad.net/ubuntu/+source/qpdf/+bug/1397413

The PDF specification says that the /Type key for nodes in the pages
dictionary (both /Page and /Pages) is required, but some PDF files
omit them. Use the presence of other keys to determine the type of
pages tree node this is if the type key is not found.
Update TODO with some small enhancements

There have been a few enhancements requested that only affect the qpdf
command line tool and that should be relatively quick to implement.
Work out the details and acknowledge these publicly in the TODO file.
TODO