Feature Request: Unsupported Formats Create a XMP Sidecar

[jonshipman]

I may end up doing a PR for this on my own time, but dropping this here as a note. For all the formats that supportsExif false (and really, we can just check for !supportsExif) we can attempt to create a xmp sidecar file. XMP files can be read by Lightroom, darktable, digiKam, infraview and others. As with anything the hard part will be testing.