Better POST parameter/content management than "Parameter Body" button

The Parameter editor is great but losing the editability of the list after clicking _Parameter Body_ is a bit frustrating. It would be nicer to have the same parameter manager if the _Content-Type_ is set to `application/x-www-form-urlencoded`.

Also Base64 might be better as a checkbox so that the transform happens on send and you don't "lose" your data.