Make "copy source post" feature run through server-side processing #140

This feature currently works with a couple of hard-coded jQuery calls. While this is a less-than ideal situation on its own, I would simply extend it with an admin-ajax call to let MLP have a look at what goes into the Advanced Translator fields (or maybe just the post content for now)

This would give us an easy opportunity to work some magic on the content (update shortcodes, URLs and so on) or give that power to plugin authors.

We could also think about an API to allow user validation of select (or any) translated content ( --> Do you want to replace this with that ? YES/NO ) and maybe even enforce it by default