Fix a pickling problems with hooke.plugin.playlist.FilterCommand.

Fix a pickling problems with hooke.plugin.playlist.FilterCommand.

Pickle doesn't like functions that aren't defined the top level of
modules (such as NoteFilterCommand's old lambda filter_fn).  Add a
warning to that effect to the FilterCommand doc string, and rearrange
to encourage the use of methods (which are less likely to be defined
on the fly than functions).

Also:
  * Added pickle queue debugging code to hooke.hooke.  This isn't the
    first tiem I've had a pickling problem.  This code (when enabled),
    makes it easier for me to track down the failing code.
  * Fix a typo in hooke.hooke.HookeRunner.run (self.ui -> hooke.ui).
  * Remove a snip of trailing whitespace in hooke.hooke.