wp_dropdown_categories should add category slug as class to each option.

[jackreichert]

Portability is important for theme developers. With more and more developers using JS for enhanced functionality of their themes it's important to offer portable selectors for their use. Category ID's tend to change in the migration process, category names do not.

While it's important to keep the class clutter low, I believe adding the slug of a category as a class to each option in wp_dropdown_categories would be very helpful for developers.

I found this modification useful for a theme I was developing and offer this patch with hope that my solution may benefit other developers.

Patch for alternative solution to problem (details in comment)

Perhaps a better solution would be to offer an option that replaces the option value with the category slug. Patch for this solution added (category-template.slug_as_value.patch)