Merged Thomas Habets 2009-01-07 XML output for "be show".

Merged Thomas Habets 2009-01-07 XML output for "be show".

I rewrote a few of his routines, e.g. generalizing
Comment.string_thread to run a caller-specified method avoided the
need for some duplicate code in Comment.xml_thread.  There was also a
reasonable reorganization of libbe.settings_object.versioned_property
because the <in_reply_to> field of the Comment.xml output was giving
me `-1' (= old settings_object.EMPTY) instead of None, even after I
had set comm.in_reply_to to None.  The rewritten versioned_property
avoids the ambiguity of UNPRIMED vs EMPTY, and avoids the stupididy of
my using EMPTY=-1 ;).