Uninitialize slices interpolator when render window part is deactivated

Summary:
The slices interpolator is initialized when the render window part is activated and connects to the slice navigation controllers, storing raw pointers to them.

To prevent null pointer accesses/crashes, the slices interpolator must also uninitialize when the render window part is deactivated.

Fixes T29380

Test Plan:

• Start Workbench
• Open Segmentation View and Standard Display if not already opened
• Close Standard Display
• Load any segmentation

This will only crash without the fix

Reviewers: kalali, O1 MITK Reviewer Group I

Reviewed By: kalali, O1 MITK Reviewer Group I

Subscribers: s349i

Maniphest Tasks: T29380

Differential Revision: https://phabricator.mitk.org/D752