Use subpanels instead of custom sections in UI

[carson-katri]

Currently the sections "Prompt", "Init Image", "Advanced Configuration", etc. use custom UI to handle dropdowns. Blender supports panels nested within panels however, and this could be a better way to separate options.

It would also allow dream textures interfaces in other areas of Blender to present parts of the form conditionally. For example, an area that doesn't make sense for init image could remove that subpanel.