Made tool manager sensitive to the visibility of the reference image
Summary: Fixes T28180
Test Plan: On toggling visibility reinit would not be required for activating the tools.
Reviewers: O1 MITK Reviewer Group I, kislinsk
Reviewed By: O1 MITK Reviewer Group I, kislinsk
Subscribers: kislinsk
Maniphest Tasks: T28180
Differential Revision: https://phabricator.mitk.org/D685