Add class to modal

Extended a config form to be able to override ui-dialog classes. Check out the MR!

I'm cool with adding a class, but it needs to have tests in all applicable locations, and schema etc.

Thanks. I see you have the schema, sorry I am on my phone at lunch and didn't look.

Also I've really tried to keep the color box implementation as a equally up to date with the UI dialogue so I would really probably like to see this implemented in both.

Even though you might not be using Colorbox I understand why you might think that should be a separate issue but I really tried to keep the two features as equal as possible in the development of the module.

No problems, it makes sense to extend this functionality to support both modal types.