Box Slicer for 2D data

[JoGaudet]

The current box slicer option for 2D data visualization is currently constrained such that the center of the box is fixed to the center of the 2D data. It would be better to allow for translation of the box slicer so it can be centered anywhere within the 2D data.

[krzywon]

Despite the different titles, this request might already be covered by #1344 and #1346. @JoGaudet - Could you take a look at those when you get a chance?

Side note - work is already being done to address some of this in PR #1919 by @butlerpd.

[butlerpd]

Indeed @krzywon and @JoGaudet, this should be fixed already in PR #1919. There remain a few other things that need to be fixed, including documentation (and the fix to issue #2482 which is mostly complete as well and should be submitted as a PR soon) before it is ready to merge. However it might be useful to test out the code from branch SlicerExtension_1344 to see if it addresses the intended use cases.