Validation API for input files

[bertfrees]

This is not the same as an API for validating data types, although the two could possibly be combined.

It could either be

• a simple function, similar to the current data-types API, to get the XML definition (RelaxNG) of a certain input media-type, which would allow clients to do anything you can do with a RelaxNG grammar, e.g. validation or completion.
• or it could be similar to the API described in Validation API for data types (options) #135 for validating data types, namely upload a file and get a validation report back. This would allow lighter clients, but is possibly more limiting.

A use case is the TTS configuration file. The new API would basically assist you with writing the config file, e.g. give you a list of supported voices. A more far-fetched use case is to validate a CSS style sheet.