Add PNG extractor
Textual information, such as author information, creator software, and a description can be stored in a PNG's tEXt, zTXt, and iTXt chunks, [1].
libexiv2 can extract EXIF information out of PNGs but does not support PNG Textual Information, see [2], however Qt's PNG handler makes this information readily available as textKeys.
[1] https://www.w3.org/TR/PNG/#11keywords [2] https://github.com/Exiv2/exiv2/issues/1343
For example, PNGs exported from Inkscape now show up as "Document Generated By: www.inkscape.org"
Some test files in exiv2 repository [3] have a long description both as EXIF Description and PNG Comment. Not sure if that is a big deal, though, as I didn't even know PNGs could have EXIF, and using both imho is unlikely.
[3] https://github.com/Exiv2/exiv2/blob/main/test/data/ReaganLargePng.png