Render window manager with custom multi widget: beta version
Summary: Changes include some Core changes that have to be reviewed.
Test Plan: manual
Reviewers: MITK Reviewer Group I, O1 MITK Core, floca
Reviewed By: MITK Reviewer Group I, O1 MITK Core, floca
Subscribers: floca
Maniphest Tasks: T25688
Differential Revision: https://phabricator.mitk.org/D175