Fixed T27267: [Image Statistics] Removal of image and planar figure from data storage does not affect statistics widget

Summary:
Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>

Fixed an bug in the MultiNodeSelectionWidget

Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>

Covered T27267 by introducing MultiSelectionWidgets, that took care of the node monitoring

also cleaned up the business logic and removed
obsolete stuff like the handcrafted planar figure
observer (it is now covered by the auto update
feature and signals of the statistics data generator

Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>

Test Plan: Test in workbench / checklist

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Maniphest Tasks: T27267

Differential Revision: https://phabricator.mitk.org/D274