Add Duplicate Button to Theme Builder Templates

## Prerequisites

- [x] I have searched for similar features requests in both open and closed tickets and cannot find a duplicate.
- [x] The feature is still missing in the latest stable version of Elementor ( Elementor Pro. )

**What problem is your feature request going to solve? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
I am frustrated there is no duplicate button on templates in the theme builder. I make a single page template and I need to make another single page template with a few modifications and it would be so much easier to duplicate the first one and then tweak it and reassign it to different conditions/pages.

**Describe the solution you'd like**
With a duplicate button {See added graphic}, I could duplicate a single page template and have the same template to be able to modify a color or image, WITHOUT having to manually recreate the single page template again.

**Describe alternatives you've considered**
I am having to recreate the single page template again and I may make a design padding or margin mistake and the template jumps or moves when going to different pages. Sure I could save it as a template, but it would be a one click duplication on a single page template and make the color change without touching anything else.

**Additional context**
Add any other context or screenshots about the feature request here.

The issue names no files or tests; start at the Theme Builder templates list and compare it with the existing save-as-template flow mentioned in the request. Done means a template can be duplicated from the Theme Builder, retain its existing design, and be modified and reassigned to different conditions or pages.

Written by the indexing model from the issue text.