Segmentation Utilities view shows gaps between multiple selection widgets

Summary:
Fix stretch policy to minimize the gap between multiple selection widgets

The size policy of a data selection widget was minimized in order
to remove the gap between multiple selection widgets.

Test Plan: manual

Reviewers: kislinsk, O1 MITK Reviewer Group I, floca

Reviewed By: O1 MITK Reviewer Group I, floca

Subscribers: kalali, kislinsk, floca

Maniphest Tasks: T27792

Differential Revision: https://phabricator.mitk.org/D421