Support XMP metadata in browser

[Marsu42]

In the browser details, you can only select from a fixed list of iptc & exif metadata variables. However, XnViewMP is able to display most xmp metadata in the image "Properties" just fine, so obviously the toolkit is there w/o calling exiftool. XMP is becoming more important as esp. the move from the legacy IPTC is clear, newer IPTC extensions are to be found in XMP tags.

Suggestion: It would be nice if we'd could insert *any* metadata from the properties panel in the usual suspect's location (tooltip, caption for thumbnails, ...) like {XMP-dc:Identifier}.