Allow comment reparenting to fix poor threading

[Otto42]

For the OCD among us, it would be super nice to be able to edit the comment_parent, to properly thread comments made in the wrong place/threading order.

Choose your own UI, but even just a numeric editor in the Quick Edit area would be a huge enhancement.

This was something that was worked on during GSoC 2010:

​https://gsoc2010.wordpress.com/matt-harzewski-comments/

Perhaps something can be derived from existing plugins, like ​Move WordPress Comments by Nicolas Kuttler and my own ​Edit Parent Comment ID.

I recently opened ticket 20292 which was closed as a duplicate of this ticket.

I've attached a patch file that adds a Comment Parent to the 'edit page' and allows a user with edit-comment privileges to enter a new integer into this field to update the database so enabling rethreading.

I've briefly tested it and it allow me to shuffle comments about changing their parents and even reverting the parent field to 0.

Updating patch against current trunk code