JSON-RPC: add possibility to specify the player to be used

This adds a new method Player.GetPlayers to JSON-RPC which lists all available players (dvdplayer, paplayer, any external players and any upnp renderers) which makes it possible to add a "playerid" parameter to Player.Open which allows JSON-RPC clients to start a file / media item with a specific player. This mostly makes sense for remote playback over UPnP.