Fix single slice confirmation using first label

Summary: Fixes T29520. The (binary) preview of a 2D interpolation is now transfered to the correct active label value before it is applied. Applying it now uses more existing functions.

Test Plan: Test the 2D interpolation in the Segmentation view for various labels (and timesteps). Confirming for a single slice should now work properly, i.e. use the correct label value.

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T29520

Differential Revision: https://phabricator.mitk.org/D810