Make text/x-mrml a subclass of application/xml

Section 3 of the MRML specification states "it is XML-based, meaning that standard parsers can be used to process the communication messages."

https://www.researchgate.net/publication/2329915_MRML_An_Extensible_Communication_Protocol_for_Interoperability_and_Benchmarking_of_Multimedia_Information_Retrieval_Systems