Summary: Add a function to compute the whole image range, if a colormap was chosen
Additionally the colormap-action has been made a batch-action, so that
multiple images can be assigned to a colormap.
Use whole pixel range only for multilabel colormap
Refactor level widget context menu and lookup table
Test Plan: manual
Reviewers: kislinsk, O1 MITK Core!
Maniphest Tasks: T23590
Differential Revision: https://phabricator.mitk.org/D172