Fix crosshair initialization

Summary:
React if the mxnmultiwidget editor is opened / closed

This allows to activate the render menu / crosshair
after all render window (widgets) have been initialized.

Test Plan:
manual testing

1. open MITK, close the StdMultiWidget
2. open the MxNMultiWidget and load a dataset
3. check if the corsshair is visible / moveable
4. check if the render window menus works
5. change the number of render windows
6. change the layout
7. check if the crosshairs are visible / moveable
8. check if the render windw menus work

Reviewers: s434n, kislinsk, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II

Reviewed By: kislinsk, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II

Subscribers: kislinsk, s434n, kalali

Maniphest Tasks: T26627

Differential Revision: https://phabricator.mitk.org/D691