TotalSeg nnUNet Confirm Segmentation button bugfix

Summary:
In both Totalseg and nnUNet, the m_FirstPreviewComputation is now set to false before tool->UpdatePreview() is called.
Also,
in SegWithPreviewTool, the m_IsPreviewGenerated set to false when tool is activated. This to avoid a case where Totalseg, without having any Preview, invokes a confirmation dialog up on exiting due to the tool's previous runs which generated previews.

Test Plan: NA

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T30392

Differential Revision: https://phabricator.mitk.org/D931