Exiv2 returns wrong XMP type for nested XMP keys
The type of a nested Exiv2 XMP key (one which contains a "/"), like Xmp.MP.RegionInfo/MPRI:Regions, is not properly detected. Exiv2 tries to find an XMP "RegionInfo/MPRI:Regions" property in the lookup table for namespace "MP". Since that doesn't exist it uses the default XmpText type. Instead, t should look for the type of the innermost property, MPRI:Regions in the example.
This bug was unearthed in a discussion about how to add new XMP namespaces and use them from pyexiv2.
#799: Fixed type-determination for nested properties,
XmpBag::read: don't do anything if the string is empty,
Added Iptc4xmpCore and Iptc4xmpExt as possible prefixes