Make selection in Properties view update on changes

Summary:
The available selectable render windows for which to show the property list in the Properties view are now updated, instead of being fixedly set on creation, when:

• a different RenderwindowPart is activated
• the current RenderWindowPart changes (like when the layout is changed in the MxN MultiWidget)

Test Plan: Switch between the StdMultiWidget and MxNMultiWidget views, change the MxN layout, observe the "Property List" combobox in the Properties view always containing only "Base data", "Data node: common", and for each currently shown render window something like "Data node: <window name>"

Reviewers: O1 MITK Reviewer Group I, kalali

Reviewed By: O1 MITK Reviewer Group I, kalali

Maniphest Tasks: T29531

Differential Revision: https://phabricator.mitk.org/D798