fdo#80927: css.office.Quickstart should be single-instance service.

Incorrectly converted in a384b21cc40818bf3c918951a086a30b5d9d8022
where SFX_IMPL_ONEINSTANCEFACTORY was used.

AFAICS it's the first converted single-instance service which implements
css::lang::XInitialization. That's kind of strange but can do its job.

sbergman@redhat.com:  Three things were necessary in order to not call the
~ShutdownIcon code too late during exit now:

* Move the relevant code from ~ShutdownIcon to ShutdownIcon::disposing.

* Add a dummy <singleton name="com.sun.star.office.theQuickstart"/> so the
  service manager will eventually dispose the (single) instance.

* In
  cppuhelper::ServiceManager::Data::Implementation::createInstanceWithArguments
  do not shortcut updateDisposeSingleton in that odd case of calling
  createInstanceWithArguments on an implementation that (effectively) is a
  singleton (as otherwise the service manager would still not dispose it).  It
  looks to me like that "return inst;" was an inadvertent leftover in
  874c481801434d4fac3c50f076bff0fe3a3988b6 "Simplify service manager's tracking
  of singletons" and wasn't intended to serve some subtle purpose.

Change-Id: Icd4d3168ec0bbb820b17ac321fe897ac9f9ce7fc