Adapt screenshot maker to support MxN MultiWidget

Summary: This is part of T29302. The Screenshot Maker view does not use hardcoded window names anymore, but dynamically gets them from the current RenderWindowPart

Test Plan: Use the Screenshot Maker view to create screenshots while either the StdMultiWidget view or the MxNMultiWidget view is open. Both should work and offer all their windows for the 2D screenshots. While the MxN view is open, the 3D screenshot section should be deactivated

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Differential Revision: https://phabricator.mitk.org/D802