Preferences, singletons

Create a Preference dialog for fonts and audio output devices

Turn Preferences and SoundOff into singletons.  No matter how many times
you request a new one, the same instance is returned.

Stop using singals on the parent() to access other instances, such as
sound and Preferences.

lib/__init__.py

@@ -2,4 +2,3 @@ from .books import Book
 from .person import PersonDialog
 from .read import EditDialog
 from .session import SessionDialog
-from .sounds import SoundOff