Add the beginnings of join game support.
Move and rename the RealTimeGameBso and implementation.  Move them to the
repository layer/package.
Implement joining a game on the backend and send out the appropriate
events when the game is starting.  The front-end logic of opening a new
window and waiting for the game to start still needs to be implemented.
Cleaned-up a bunch of event verification code in RealTimeGame.  Move
some code around from the bso layer to the command/query layer.