Added hooke.plugin.playlist.ApplyCommandStack and related changes.

Highlights:
* Added test/apply_command_stack.py testing the new command.
* Added hooke.plugin.playlist.PlaylistCommand mirroring CurveCommand.
* Added hooke.plugin.playlist.AddingPlaylistCommand mirroring
  ColumnAddingCommand.
* Renamed CommandStack._execute -> .execute_command since it may be
  used externally.
* Changed assorted log messages to use CommandMessage string-ification.
* Set Hooke.engine to None in the UI process.
* Set ._index during NoteIndexList.items() iteration.