Add MxN to Default build configuration

Summary: Adds the MxN MultiWidget to the default build configuration (and by extension, the workbench release config)

Test Plan: Build MITK with the Default or Workbench Release config. MxN should appear in the available views

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Maniphest Tasks: T29568

Differential Revision: https://phabricator.mitk.org/D823