Apply level window to multiple selected images
Summary: Changes to allow setting an option in the context menu of the level window slider to work on multiple selected nodes (selected in the data manager).
Test Plan: manual
Reviewers: kislinsk, O1 MITK Core, floca
Reviewed By: kislinsk, O1 MITK Core, floca
Subscribers: kislinsk
Maniphest Tasks: T25483
Differential Revision: https://phabricator.mitk.org/D181