Draft: Restore open document selector

This is a suggestion for restoring the "recent documents" open widget that was recently removed, as discussed in #563 (closed).

This functionality has had a turbulent history. It came in to existence in the very ancient times. It then got removed in 27f54a9a for gedit 3.36, resurrected in fa587e03 for gedit 41 (and 40.1?), and then removed again in 5164375a for gedit 43.

It adds a bit of code, but it is a welcome piece of functionality among long term gedit users. So hopefully, we can find a way to bring it back and keep it.

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