Preventing NotificationService from registering if unused

Until now, once `libqtile.widget` was imported, a new `NotificationService` was
created and registered as a DBus Notification daemon, and thus making impossible for any other notification daemon to register.

This patch prevents `NotificationManager.service` (a `NotificationService` instance) to be initialized until further call of `register()` method, which basically avoid registering as a notification daemon when the configuration does not uses any notification widget.