Propagate sys.exit(code) up the call stack to the shell

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.

src/Main/MainGui.cpp

@@ -236,8 +236,8 @@ int main( int argc, char ** argv )
         else
             App::Application::runApplication();
     }
-    catch (const Base::SystemExitException&) {
-        exit(0);
+    catch (const Base::SystemExitException& e) {
+        exit(e.getExitCode());
     }
     catch (const Base::Exception& e) {
         e.ReportException();