JSON-RPC: Extend EPG functions to be more convenient to client applications

Added EPG information to PVR.GetChannels, so that the active broadcasts for a long channel list can be retrieved much faster.
Also added a start- and enddate filter to PVR.GetBroadcasts, to reduce the amount of unneeded data.

I am not completely sure, that changing the serialization of CPVRChannel does not break anything else, but testing did not reveal any problems.

This is also a plea for ideas on how to filter the fields of "broadcastnow" and "broadcastnext", because likely applications will only need a couple of them.