Handle Server Shutdown Better

[Flozza]

When the server requests a shutdown, in theory the client should exit, too.

There are two issues here:

1. The client does not properly shut down right now (e.g. the GUI does not exit, etc.). This should be fixed regardless of anything else: If an exit is requested, the GUI should shut down as well.
2. When the server shuts down, the better way would be to just close the connections and try reconnecting, not shutting down at all. During a CTF it might happen that the server restarts and usually we want the client to keep collection flags and send them again once the server is up again.

[Flozza]

1. is now in issue #42

[Flozza]

This is fixed already...