SystemExitException is instantiated and thrown by Base::Interpreter
whenever the python interpreter executes a sys.exit(<exitCode>) call.
However, in the current implementation, the SystemExitException
effectively swallows the <exitCode> hence under this change,
SystemExitException has been extended to save the <exitCode> passed to
the sys.exit() call when it is instantiated and provides a new
getExitCode() accessor method to retrieve the <exitCode> in a
try/catch block. MainGui.cpp was change to pass the corresponding
<exitCode> to the system exit() call so it is accessible in the shell.
