Refactor|libappfw|GuiWidget: Added a slot for deleting later

Also added a guiFind() utility method and renamed the clipped()
method for clarity.
Fixed|libappfw: Handling notifications owned by other widgets

If a notification is owned by a widget prior to showing,
NotificationWidget is supposed to only temporarily take ownership
of the notification. This requires observing the original parents
and seeing if they get deleted.

Todo: Add a utility class for this.
Tutorial: Introduce notifications

Using dummy notification area and alert.
Refactor|libappfw: Renamed NotificationWidget

This is a notification area, not an individual notification.