plug-ins: Load PSP EXIF and XMP metadata

@Wormnest Hi! Whenever you have time, could you take a look and let me know what you think?

XMP wasn't documented in the PSP 8 specs, but the sample image had it and it appears to be just a 32-bit length value and then the XMP data.

EXIF loads correctly, but GEXIV2 throws a lot of warnings. I'm not sure what could be done about it.

Sample Image at #49 (comment 903743)