Reworkd AbstractNodeSelectionWidget and introduced node changed observers
Summary:
Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>
Started unit test for AbstractNodeSelectionWidget
Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>
Test Plan: unit test and code review
Reviewers: MITK Reviewer Group I, kalali
Reviewed By: kalali
Maniphest Tasks: T27064
Differential Revision: https://phabricator.mitk.org/D250