Panel border for Inset

[will-moore]

See https://forum.image.sc/t/feature-request-for-omero-figure-iviewer-and-insight/57817

Add a border to a Panel, so that when it overlaps other panels (e.g. showing Inset), the edge can be seen.

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/feature-request-for-omero-figure-iviewer-and-insight/57817/2