Add auto-discovery of new *.css files in the "User Styles" folder to allow importing them

[Drugoy]

If the user puts new .css files to that folder - the add-on should discover them and prompt user to allow using them. Maybe without a prompt it still will be okay.

[alopepeo]

Would be great if you can add this feature for next version of USM. You have a lot of work, I think, so if you can't do it for version 1.1, would be great that you can implement it for the next version (1.2 I suppose).

Thanks!