Show pixel value for mxnmultiwidget

Summary:
Update the crosshair with a new slice

When scrolling the SliceNavigationController uses SendSlice and invokes
a GeometrySliceEvent when the slice stepper changed.
This event has to be connected to the newly introduced function to
correctly update the slice for the crosshair planes.

React on the GeometryUpdateEvent of the SliceNavigationController

This is required for the mxnMultiWidget since a crosshair reset
will not trigger the already connected GeometrySliceEvent.
Reacting on the GeometrySliceEvent was enough for the StdMultiWidget
since the SliceNavigationController were connected and a crosshair reset
would trigger a slice change in other render windows.
For the mxnMultiWidget this is not the case, so we need to listen to the
GeometryUpdateEvent additionally.

Clean up some classes

Test Plan:
manual testing

• open MITK, close the StdDisplay, open the mxnDisplay
• open the Pixel value view, load an image and set the crosshair to a different position
  • also scroll through the slices
• see how the pixel value changes accordingly

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

Reviewed By: s434n, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II

Subscribers: floca, kalali, s434n

Maniphest Tasks: T29385

Differential Revision: https://phabricator.mitk.org/D756