JSON-RPC: support for fetching PVR timers and recordings

This is an improvement of #4198 with the necessary code for proper JSON-RPC integration and the definition of the JSON schema for the new methods.

I have no clue what the available fields mean so I just added them to the best of my imagination. Let me know if I got anything wrong @opdenkamp.

This is completely untested as I don't have a PVR setup and didn't have the time to setup the PVR demo addon. So feedback from @joethefox, @Tolriq or whoever else knows how to use the JSON-RPC API is most welcome.