jsonrpc: add support for multiple items to Playlist.Add/Insert

As requested at DevCon I've added support for multiple items to Playlist.Add and Playlist.Insert. The change is implemented in the existing methods without breaking backwards compatibility. The implementation works in a way that it returns OK if at least one of the passed in items was successfully processed. It only returns "Invalid params" if all of the items failed. I'm happy to change that behaviour to completely fail if one of the items can't be handled.