Interrupt signal not handled properly for cli and gtk interfaces

When sending an interrupt signal to checkbox, ie ctrl-c, the behavior doesn't actually interrupt the application. For example, in the command line interface, sending the signal after the introduction results in the application proceeding with the subsequent information gathering step. And, in the gtk interface, the application doesn't stop but then pressing next suddenly stops with several repetitive KeyboardInterrupt stack traces. Instead, the behavior should be as expected and the application should simply be interrupted. Ideally, the application should catch the signal and attempt to flush cached information such as the persisted information.