Summary: Add a function to compute the whole image range, if a colormap was chosen

Summary:
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: O1 MITK Core, MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Core, MITK Reviewer Group I, kislinsk

Subscribers: kislinsk, kalali

Maniphest Tasks: T23590

Differential Revision: https://phabricator.mitk.org/D172