Immediately apply "Show only selected nodes"

Summary:
Fix T29106
Switching on the "Show only selected nodes" checkbox in the Windows -> Preferences -> Segmentation should now
take affect once the "OK" button is pressed.

Changes:

void QmitkSegmentationView::ApplyDisplayOptions(mitk::DataNode* node) has been changed.

Toggling the "Show only selected nodes" checkbox in the Windows -> Prefences -> Segmentation page changes the variable
m_SelectionMode in QmitkSegmentationView.

Flow of information:

Pressing "OK" in the Preferences page triggers a QmitkSegmentationView::OnPreferencesChanged call, which in
turn subsequently calls the ApplyDisplayOptions function. This function is where the change is introduced.

In the function, if m_SelectionMode is true only the working node is set to visible. All other segmentation nodes are set to invisible.

Test Plan:
Switching on the "Show only selected nodes" checkbox in the Windows -> Preferences -> Segmentation should now
take affect once the "OK" button is pressed.

Reviewers: O1 MITK Reviewer Group I, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T29106

Differential Revision: https://phabricator.mitk.org/D687