Added an option to automatically exit after closing the MASSIVE Desktop.
It would have been easier to just hard-code the auto-exit to keep
behaviour consistent with previous Launcher versions, but I have
implemented an option for this, under the assumption that disabling
auto-exit might be preferred by some users, especially when using
the CVL tab.

The reason why enabling auto-exit was a high priority for me, is
because I was having to click 5 times to end my MASSIVE Desktop
session:

1. One click on the "Exit MASSIVE Desktop" KDE panel button.
2. One click on the KDialog's "Continue" button.
3. One click on TurboVNC's "Connection Closed" dialog's "OK" button
(which only occurs on Mac and Windows, not on Linux/X11).
4. One click on the "Stop the Desktop?" dialog
5. One click on the Launcher's Cancel button to exit the Launcher.

Too many clicks for me, hence the auto-exit functionality!