Resolve "Metadata Modules"

This MR comprises a first draft for the "Editions (manuscripts)" module as a first module addition for the TextAPI.

In order to identify the necessary fields, I talked with Uwe about the most important metadata fields in the SUB TEI schema and derived the most essential keys from it (i.e. the metadata that probably most projects want to provide for a manuscript edition).

As stated this is only a first draft and we certainly can talk about cardinalities and the like.

Closes #11 (closed) and closes #10 (closed).