Add UI for layered fog

[nib9888]

#1428 needs a UI, and since this would be fairly complex it's being split from the functionality and worked on separately. Here's a potential mockup:

Labelled:

There has been some discussion on the discord about this UI, and this issue can serve as a tracker for work related to this.
Hopefully @ShirleyNekoDev can work some magic on this, and open a PR when we're closer to a working prototype.