Transaction cancel

Adds the ability to cancel transactions with the Escape key.

The TransactionCheckEvent has a new 'cancel' member indicating whether or not the user wants to cancel the transaction.  This is set if the user hits Escape when the dialog window is open.  GameLogic's transactionCheckEventHandler looks for this and queues a TransactionCancelEvent if the user indicated they want to cancel.  The main loop unpauses the game and the human game view removes the window when this happens.

Resolves #67 .