Added SSM compatibility

There you go mate.

I have added 2 fix:

1 - Added the same CSS fixed that I used in save_and_return (i.e. <https://github.com/Solutions-Nitriques/save_and_return/blob/master/extension.driver.php#L213> vs. <https://github.com/DeuxHuitHuit/duplicate_entry/blob/master/extension.driver.php#L66>)
2 - Since your extensions uses JS to create the button, I simply added a simple if clause to check if we are in the SSM (i.e. <https://github.com/DeuxHuitHuit/duplicate_entry/blob/master/assets/duplicate_entry.js#L51>)

You are free to use both or only one.