LiveWire Closing Contour improvements

Summary:
The LiveWire tool used a filter function to calculate the closure contour which was not necessary since the contour was just a straight line.
The straight line contour is replaced by just two vertices that are connected. Additionally, the user has the possibility to change the behaviour of the closure contour from a straight line to snapping.
This is possible with a checkbox in the GUI which also can be toggled with the space key as a shortcut.
Fixes T28959

Test Plan: manual

Reviewers: kislinsk, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Maniphest Tasks: T28959

Differential Revision: https://phabricator.mitk.org/D590