Issue #4733: Add Neutral Gray fill option for layers

This attempts to resolve #4733 (closed) by adding a new Neutral/50% Gray fill option (after Background Color).

It mostly saves users an extra step or two if they use neutral gray a lot (e.g. dodging/burning as the initial report says). I'm not sure if it's sufficiently unique/useful to warrant a pre-set; hopefully this merge request will help resolve the issue one way or the other. 😄