Updated gui.panel.note and simplified hooke.plugin.note.

Updated gui.panel.note and simplified hooke.plugin.note.

Previous hooke.plugin.note had assumed curve.info['note'] was a list
of strings.  The current implementation is simpler (and class
agnostic) with get/set commands.

NotePanel now works as a UI to the current curve's .info['note'], but
it has exposed some issues with .info in Playlist.flatten().

Also removed some outdated binding cruft from gui/__init__.py (now
handled by the panels themselves) and an outdated _callback setting
from panel.commands (it's set in the base Panel.__init__()).