Allow searching in the "Show Original" window

Please add, or no longer suppress, the ability to do a find/search in the “Show Original” window.

(Originally posted by jerm on GitHub.)

It looks like this is possible, but the default UI isn’t available (that tiny overlay with the text field and up/down arrows) so I’d need to roll something. I think this would apply to the print window as well though, so I’ll see what I can do!

(Originally posted by bengotow on GitHub.)