Show rotation axis in the image viewer

[dagewa]

I'm working on the VMXi thaumatin data (dials.data get vmxi_thaumtin). Just after import I see this image:

I see the backstop shadow is vertical and I wonder if I have to mask it, because maybe the rotation axis is horizontal. Unfortunately I have no way to see the rotation axis direction.

However, this feature is available in dials.image_viewer:

and looking at that I see that the axis is aligned to the shadow and there is no need for me to mask.

Please add the option to show the rotation axis in DUI2, so I don't have to go back to the dials.image_viewer.

[dagewa]

Here is the code from dials.image_viewer that is responsible for figuring out the axis display:
https://github.com/dials/dials/blob/044caa37325beec1d18a00098ebdeff35ea0a358/src/dials/util/image_viewer/spotfinder_frame.py#L1336-L1401