More progress on networking code

NioSession.createServer received a similar treatment to
NioSession.createClient and IncompleteNioServerSession inherits from a
common class that IncompleteNioClientSession (the old
IncompleteNioSession) now is also a child of. Parts of
DirectConnectModel have been abstracted for other networking scene
models to share. AwaitingClientScene will now start listening on the
default port upon being opened. WifiDirectSelectionScene now
automatically connects to the first discovered WiFi Direct broadcasting
device. IncompleteNioSessions are now canceled when the networking
scenes are closed before a connection is established. A plain
BattleScene will open when a connection is successfully established in
any one of the networking scenes. BattleScene now sets its clear color
to white.