Support identification of PNG additional data

[brachna]

PNG can have additional data inside it (I believe Hydrus' own png plugins use it).

I know of 2 examples of this:

1. Koikatsu exports pngs with scene data information.
2. ComfyUI exports generated pngs with their workflows inside them (https://github.com/comfyanonymous/ComfyUI/discussions/1533) There might be more of course.

Right now Hydrus doesn't detect it. When I filter Koikatsu scene data pngs and add these tags: system:no embedded metadata system:no exif They still all show up.

It's very important for duplicate processing. So when you encounter such PNG you have no clue if it does contain additional data, you just see it's larger. Adding support for this will also make it easy to find such files.

So my request is for Hydrus to detect such additional png data and show if it's there in duplicate processing decision. Like "this png file contains additional data". Also add system tag like "system:additional data".