P: Add Lights-out viewing mode

Hi all,

I have recently started using LR Desktop in earnest, transitioning from LR Classic. The recent updates are excellent. But there are some simple things I miss, such as the 'Lights out' option in Classic.

Also, I don't see a way of comparing more than two images at at time, when culling/comparing images for example.

Only small things, but it would be nice to have them.