Remove Restriction of XMP Namespaces

[ola-github]

Currently the mapper supports the following XMP namespaces:

• http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/
• http://iptc.org/std/Iptc4xmpExt/2008-02-29/
• http://purl.org/dc/elements/1.1/
• http://ns.adobe.com/photoshop/1.0/
• http://ns.adobe.com/xap/1.0/rights/
• http://ns.useplus.org/ldf/xmp/1.0/ This set of namespaces is explicitly defined in the xsd.

With a related validation functionality the metadata mapper could support all namespaces provided by the underlying exiftool. Certain validation functionalities already exist and may be improved by this feature.