json-rpc: add possibility to force using the default player through Player.Open

This adds `default` as a valid value for the `playercoreid` property in the Player.Open call to give JSON-RPC clients a way to start playback of an item with Kodi's default player. Right now when e.g. a movie is playing on a remote UPnP renderer and a JSON-RPC client calls Player.Open with another movie and doesn't provide the `playercoreid` option the next movie will also start on the remote UPnP renderer. There's no way to force the movie to play in Kodi's default player (whatever that may be).

@Tolriq: Please let me know if this helps to improve the situation.