We no longer have a single notification actor that is either displayed
as banner or reparented to the summary depending on state - both the
lock screen and the notification section of the message list create
their own UI based on the information attached to the notification
object. Adding to this that different representations of a notification
may now exist simultaneously (as they are included in the message list
immediately rather than after the banner has been displayed), it no
longer makes sense to keep the banner actor in the notification itself.
Add a new NotificationBanner class that provides a separate banner
implementation based on the message list's NotificationMessage that will
soon replace the existing notification banners.
53b3d2a6c2