for listening for commands from network;
transfer fields; attempts to get "play" to play, etc.
Some from Stuart: PpCmd uses string object;
passing PpCmd commands by transfer-field
from master to slaves.

according to whether msg() or warn() was called.

onWindowStartGL() returns void.

Implement PpCmd interface for serializing commands for network transport.

though not to process any commands after command-line startup yet.