XMP support
***********

XMP support is controlled with the --enable-xmp and 
--disable-xmp options of the configure script. Building Exiv2
with XMP support requires the Expat XMP parser library, which
is widely available. The --with-expat configure option can
be used if it is installed in a non-standard directory.

Top-level Exiv2 classes to access XMP metadata are XmpData,
Xmpdatum and XmpKey. They work similar to the corresponding 
Exif and IPTC classes. The property-repository is XmpProperties.
In addition to the expected new members, class Image also
has an interface to access the raw XMP packet.

All XMP value types are supported: Simple types, structures,
arrays, property qualifiers and language alternatives.

XMP properties are accessed through keys of the form
"Xmp.<Prefix>.<PropertyPath>", where <Prefix> is the preferred 
(or rather, registered) prefix for a schema namespace and 
<PropertyPath> is the path of the XMP node. In its most basic
form, to address simple properties, <PropertyPath> is the name
of the property. In general, <PropertyPath> can be used to 
address any XMP node, including array items, structure fields
qualifiers and deeply nested properties.

Any properties in known namespaces are supported and additional
namespaces can be registered.

The specialized Exiv2 values XmpArrayValue and LangAltValue are
provided to simplify the use of XMP properties.

See xmpsample.cpp for examples of how to set various types of
XMP properties.

Note: Unlike Exif and IPTC tags, XMP properties do not have
a tag number.

Todo: Conversion between XMP and Exif/IPTC metadata.