Implement metadata according to the generic TextAPI

Goal: a reference implementation according to the specs given in the generic TextAPI

Pls refer to: generic TextAPI

TODO:

• remove rendering for x-prefix (project-specific API eXtensions)
• render metadata object provided by the manifest object holding arbitrary, project specific data
• keep item-sections from being rendered if no data is delivered => (cardinality "?" / "*")
• adjust the (global) config options
• update the README accordingly

Decisions to be made:

• should everything be rendered with a cardinality of either "1" and / or "+" (if not, what should be rendered?)
• which metadata fields should be configurable (imho everything with cardiality "1" and / or "+" as far as it is about to being rendered)
• in which order should we display the single fields

Related:

• Provide license information for images
• Provide license information for the text
• Move x- prefixed info to Metadata Object in back end