Restructured Checker class to get rid of 'ext' table.

Links are now either in 'todo' or 'done', and ext links
are hadled more like local links except that no further
links are gathered (and sometimes they aren't checked,
e.g. for mailto and news URLs).  The -x option reverses
its meaning: it disables checking of ext links (they are
moved to 'done' without checking).  A new 'errors' table
collects pages with bad links as we go -- redundant,
but useful for the GUI version which needs to report
this as we go.  Some new methods, including reset().
New checkpoint format.

Adapted the GUI to the changes in the Checker class.
Added Quit and "Start over" buttons, and a checkbox
to disable checking external links.  The details
window now also shows bad links emanating from the
selected page.  Miscellaneous small chages.