Move the 'GetTopLayerNode'-function to the data storage

Summary: Refactoring: Remove leading namespaces

Test Plan: manual testing

Reviewers: kislinsk, O1 MITK Core, floca

Reviewed By: kislinsk, O1 MITK Core, floca

Subscribers: floca, kalali

Maniphest Tasks: T24173

Differential Revision: https://phabricator.mitk.org/D148