Added Bug.extra_strings to support add-on functionality, e.g. `be tag`.

Added Bug.extra_strings to support add-on functionality, e.g. `be tag`.

Versioned properties whose data is a mutable type are tricky, since
the simple comparisons we'd been using in
libbe.properties.change_hook_property don't work for mutables.  For
now, we avoid that problem by assuming a change happened whenever a
mutable property is set.  change_hook_property is a bit untidy at the
moment while I work out how to deal with mutables.

As an example of using Bug.extra_strings to patch on some useful
functionality, I've written becommands/tag.py.  I'd suggest future
add-ons (e.g. becommands/depend.py?) use the "<LABEL>:<value>" string
format to keep it easy to sort out which strings belong to which
add-ons.  tag.py is still missing command line tag-removal and
tag-searching for `be list'.  Perhaps something like

  be list --extra-strings TAG:<your-tag>,TAG:<another-tag>,DEPEND:<bug-id>

would be good, although it would requre escaping commas from the tags,
or refusing to allow commas in the tags...

libbe.properties.ValueCheckError also got a minor update so the
printed error message makes sense when raised with allowed being an
iterable (i.e. check_property) or a function
(e.g. fn_checked_property).

All of this digging around turned up a really buggy
libbe.bugdir.MultipleBugMatches.  Obviously I had never actually
called it before :p.  Should be fixed now.

libbe.comment._set_comment_body has also been normalized to match the
suggested change_hook interface: change_hook(self, old, new).
Although, I'm not sure why it hadn't been causing obvious problems
before, so maybe I'm misunderstanding something about that.