OGR VRT: update ogrvrt.xsd XML schema and move it to GDAL data directory so that it gets installed; add validation of OGR VRT document against the schema (if libXML2 available); can be disabled by setting GDAL_XML_VALIDATION configuration option to NO

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27000 f0d54148-0727-0410-94bb-9a71ac55c965