Correctly show preview nodes

Summary: Set the label of the preview node to visible

Test Plan:
manual UI testing

• open MITK, open the Segmentation plugin view
• load an image, create a segmentation node
• create a label and hide it
• open e.g. the Threshold-tool to create a preview

-> see how the preview can be seen (outline)

Reviewers: kislinsk, s434n, O1 MITK Reviewer Group I, floca

Reviewed By: s434n, O1 MITK Reviewer Group I, floca

Subscribers: floca, kalali, s434n, kislinsk

Maniphest Tasks: T29142

Differential Revision: https://phabricator.mitk.org/D651