* Amended GetAll() on IDataTypeService to return an empty collection rather than null.
* Added a ClearSessionValue method to ISessionManager (given you can no longer set a value to null).
* Allow for null values in a StatefulNotification.
* Removed obsoletion of synchronous messages on TreeControllerBase.
* Fixed further CS8620 warnings in core project.
* Further fix to nullable warning.
* Aligned nullablility of retreiving tree nodes and menus, synchronously or asynchronously (such that we no longer can get null values, always empty collection objects).
* Gather all notifications in Umbraco.Cms.Core.Notifications
* Rename notifications to match convention
* Move and rename missed notifications
* Move the three remaining public notification into Umbraco.Cms.Core.Notifications
