Metadata export should support xsd attributes

[DavidBlaa]

after v 2.17 it is now possible to read in and use attributes from the xsd, the export must now also be adapted.

import / Export Metadata

• include attributes defined as refs (datacite - name idenftifer)
• datacite- xs:lang attribute is missing
• include attributes in mapping file during import
• add parameters to export
• show default values ui
• show validation results in ui

UI

• Metadata: Attributes of container elements don`t appear in the metadata form #1263