ADD new hook additionalmetadatafilter allowing processing metadata on write

git-svn-id: http://svn.montala.net/svn/resourcespace@4849 c08608d7-6e46-0410-86ca-f2a6f1370df5
FIX stop truncating e.g. iptc:CopyrightNotice field in metadata_write

git-svn-id: http://svn.montala.net/svn/resourcespace@4850 c08608d7-6e46-0410-86ca-f2a6f1370df5