The toaster-custom-images layer is the "special", Toaster-only layer that provides custom images created with Toaster. Currently, we are treating this layer like any other layer. In particular, we allow users to remove it from the project. We should probably not allow users to remove that layer from a project, otherwise things become rather odd. For example, right now, if you remove it, it will be added again the moment you create a custom image. You can also remove it and build existing custom images without problems. These differences in behaviour between normal layers and the toaster-custom-images layer are bound to confuse people. The attached design document suggests how to deal with this layer.
Note that although the design document calls this layer "meta-custom-images", the current name (toaster-custom-images) should be perfectly fine.
Created attachment 3003 [details] Design - The meta-custom-images layer