Rewrote a minor piece of protocol code that required clients to call
Game.valueOf(), which should never be necessary (I should really split
the client and server packages into separate Maven modules). Commands
returned by the "Unknown command" message are now always uppercase.
Implemented a session LEAVE server command, added a menu bar for easy
access to session- and game-related functions. Created a number of
dialog prototypes (NetworkedDialog class and subclasses). Added a
blocking request function to the ServerUplink class.