Monitor only selected nodes
Summary:
Use observer only for working nodes
Adding and removing "visible"-property observer only for the current
working nodes reduces code bloat.
Allow drawing without the reference node being visible
Enable UI elements only if the selection is valid
Test Plan: manual testing (see description in T28903#232375)
Reviewers: floca, O1 MITK Reviewer Group I
Reviewed By: floca, O1 MITK Reviewer Group I
Subscribers: kalali, floca
Maniphest Tasks: T28903
Differential Revision: https://phabricator.mitk.org/D585