This commit is for CVLFAB-471.

self.EndModal(0) has been added to new_version_alert_dialog's onOK method,
before self.Destroy(), to ensure that the modal session is closed.

The recently uncommented sys.exit(1) in new_version_alert_dialog's onOK
method has been removed, because we want launcher.py to write the log
message before exiting.

new_version_alert_dialog's onClose and OnClose functions (which were not
mapped to any events), have been removed.

The logger_debug, logger_warning and logger_error
functions no longer use wx.CallAfter(...) if they are run from the MainThread.
The logger_debug("Failed version number check.") was not making it into
~/.MASSIVE_Launcher_debug_log.txt, because the Launcher was exiting before the
the GUI event queue processed the logging event.  We only need to use wx.CallAfter
when we are writing to the debug window from the Logging thread.