Show different time points of data in MxNMultiWidget

Summary:
Enable time slicing for MxNMultiWidget

The time navigation controller is a global instance and thus
the time slice will be changed in all visible render windows
simultaneously.

Remove hard-coded render window access

This will simplify the MxNMultiWidget integration process
and can help to highlight the similarities between different
multi widgets.

Test Plan:
manual testing

• open 3D + t image
• open MxNMultiWidget
• create multiple render windows
• change values of the image navigator and observe behavior
• also load multiple 3D (+ t ) images

Reviewers: kleina, O3 MITK Reviewer Group II, floca

Reviewed By: O3 MITK Reviewer Group II, floca

Subscribers: kalali, kleina

Maniphest Tasks: T26633

Differential Revision: https://phabricator.mitk.org/D552