Implementation validiation mechanism

[ClemensBeck]

1. Extend Kitodo.Presentation extension to provide general validator functionalities
   • [x] Reusable validation mechanisms e.g. AbstractValidator and AbstractValidatorStack
   • [x] Implementation of validators, e.g., XMLSchemesValidator, for merging XML schemas into a single schema and validating against it
   • [x] Customisable validation middleware for validating against DOMDocumentValidationStack with JSON result
   • [x] Documentation
   • [x] Pull Request https://github.com/kitodo/kitodo-presentation/pull/1313
2. Extend DFG-Viewer extension with "DFG-Viewer" specific validation using the implementations of 1.
   • Development Branch: https://github.com/markusweigelt/dfg-viewer/tree/validator
   • [x] Add specific configuration of validation middleware for DFG-Viewer
   • [ ] Implementation and testing of classes for application profile validation (Basis is the METS application profile 2.3.1 in pdf form)
   • [ ] Finalize DFG-Viewer XML Schema
   • [ ] Frontend to visualise the errors or success of validation
   • [ ] Documentation

[markusweigelt]

@ClemensBeck I took the liberty of adjusting the description of the issue instead of creating a new one. :)

[ClemensBeck]

@markusweigelt With pleasure, many thanks!