tag --remove now returns bug.settings["extra_strings"] to EMPTY.

tag --remove now returns bug.settings["extra_strings"] to EMPTY.

extra_strings returns to a defaulting property from a cached/generator
property, with the help of the new, mutable defaults.  Lots of
deepcopies avoid mutable default uncertainty too ;).  And
copy.deepcopy([]) should be pretty cheap.

tag --remove had previously left settings["extra_strings"] as [],
which polluted the bug's values file.  Now the improved
defaulting_property notices a return to the default [], and sets the
internally stored value to EMPTY.

I struggled with creating a more intuitive way to notice changes to
extra_strings than the
  tmp = bug.extra_strings
  <work on tmp>
  bug.extra_strings = tmp
but didn't have any luck.  The problem seems to be that if you only
hand out copies of your default, you don't have any pointers to what
you handed out to check for changes.  On the other hand, if you hand
out your original default, any external changes will _change_ your
original default.  I suppose you could only hand out copies, but keep
a list of all copies handed out, but that sounds like a disaster.
Reassigning is easy enough.