Add explicit event for duplicating elements.

[myleshyson]

Hey! I need to add some logic specific to duplication, and noticed that there's no events specific to duplicating like there are for other actions like saving and deleting.

It looks like I can maybe watch out for the duplicateOf property on save, but it makes me nervous to rely on a property instead of an explicit action. I think it would be much nicer and less error prone to have an explicit event to listen for.

What do ya'll think of adding these two events to craft\services\Elements?

Elements::EVENT_BEFORE_DUPLICATE_ELEMENT
Elements::EVENT_AFTER_DUPLICATE_ELEMENT

I don't mind opening a pull request for it if this is on the table.