implement subscriptions config feature (issue #58)

  * implemented the intrinsic subscriptions/ignore feature
  * added LinJam::ConfigureSubscriptions() method
  * added LinJam::UpdateRemoteUserState() method
  * added NJClient::MAX_USERNAME_LEN , MAX_CHANNELNAME_LEN constants
  * added NJClient::ConnectionStatus , SubscribeMode , SaveMultitrack , SaveMixdown enums
  * added NJClient::GetLocalChannelName() convienence method
  * added Subscriptions and Subscription GUI classes
  * implemented LinJamConfig::valueTreeChildAdded() , valueTreeChildRemoved() methods
  * removed Channel::renameChannel() method