Make interpolation work with arbitrary windows

Summary: This is based on T29362 and attempts to remove any hardcoded references to certain render windows. Instead, one can give arbitrary windows (e.g. MxN windows) to the interpolator. The interpolator is now updated every time a new RenderWindowPart is activated, or the current one changed, to ensure changes from the MxN layout are considered by the interpolator.

Test Plan: Use the StdMultiWidget and the MxNMultiWidget to create interpolatable segmentations in various view directions and see if the interpolation correctly previews for them, and correctly fills out the segmentation when confirmed. Beware: T29520 may lead to wrong labels when confirming single slices, but this error has existed previous to this differential.

Reviewers: O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, kalali, kislinsk

Reviewed By: O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T29362

Differential Revision: https://phabricator.mitk.org/D795