Added a event for when the GUI is closed.

We need to send the disconnect event to the hub when the GUI is closed. The onDisconnect methods in DeviceListener will handle the event.
This should change when we come up with a protocol on Saturday. I think that each message should have a header that, besides indicating the size, should say what type of message it is (for example, a 'pose' type or 'orientation' type, or 'disconnect' type, etc...). As a temporary fix, this is okay.