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/session.py

@@ -114,7 +114,7 @@ class SessionDialog(QDialog, Ui_Dialog):
         query = QSqlQuery()
         query.prepare(
             "UPDATE sessions "
-            "SET start=:start , SET stop=:stop, SET notes=:notes "
+            "SET start=:start, stop=:stop, notes=:notes "
             "WHERE sesion_id = :session_id"
         )
         query.bindValue(":session_id", self.session_id)