Support layout icons/screenshots

I'd love the see the added support for layout icons. These would make it much easier for people to select a layout and know how it's going to look, compared with the current system of selecting a layout name from a drop-down field.

Here are some examples of what I mean:

Other suggestions (originally from here)

Possible techniques:

• Maybe adding image files with the same name in the templates folder?
• Maybe a config option that allows to toggle between Select modes A. Simple Select B. Images.
• Use a default image thumbnail for a template without an image file.
• Visually, a check/radiobox plus image would be enough. Although there might be fancier/prettier things.
• Use an approach like the clean names, where a path to the thumbnail image is provided in the comment of the template.