FEATURE: Allow "External Editor" to open data without conversion.

[AlexMax]

SLADE Version: 3.2.1 Operating System: Windows

Issue Details: Occasionally, I would like to use External Editor to open data without first converting it. My use case is that I'd vastly prefer to open various graphic files in Hex Workshop where I have a bunch of struct definitions set up instead of relying on the internal hex viewer. However, when I configure Hex Workshop, I end up getting a PNG file instead of whatever the original lump graphic was.