Fix MxN UI for MacOS

Summary: There are two parts here: the data selection and view orientation. By default, QMenus are not rendered inside the application on MacOS, but shown in the global menu bar at the top of the screen. This is problematic for the MxN UI, since there are multiple menus (one for each window), which get lost since only one is ever shown. That's why we need to specify our QMenuBar as not-native. The view orientation was simply too small, since on MacOS there is a checkmark in the dropdown list that takes up space of the UI. I was not able to disable this or cleanly adapt the size of the list, so the width now has a hardcoded minimum. This is not perfect, but at least makes MxN usable on Mac. Fixes T30192

Test Plan: Open the MxN view. All UI elements (Data menu, synchronization-toggle, slice navigation, view orientation) should be visible.

Reviewers: O3 MITK Reviewer Group II, floca

Reviewed By: O3 MITK Reviewer Group II, floca

Subscribers: floca

Maniphest Tasks: T30192

Differential Revision: https://phabricator.mitk.org/D873