sfx2: ensure that no timers are running after SfxApplication dies

Timers may access global data that's tied to SfxApplication, and that
seems difficult to fix or prevent; the SfxApplication is deleted
before the main thread gets around to handling the PostUserEvent
sent by Application::Quit().

log from xmloff_unoapi:

==32095== Invalid read of size 8
==32095==    at 0x934D245: Font::Font(Font const&) (font.cxx:239)
==32095==    by 0x1FCEABDF: ImpEditEngine::FormatDoc() (impedit3.cxx:381)
[...]
==32095==    by 0x1ED0F894: SdDrawDocument::LinkStubOnlineSpellingHdl(void*, void*) (drawdoc4.cxx:826)
==32095==    by 0x911FECD: Link::Call(void*) const (link.hxx:123)
==32095==    by 0x914F700: Timer::Timeout() (timer.cxx:224)
==32095==    by 0x914F394: Timer::ImplTimerCallbackProc() (timer.cxx:121)
==32095==    by 0x170B350A: SalTimer::CallCallback() (saltimer.hxx:53)
==32095==    by 0x170B26C6: SvpSalInstance::CheckTimeout(bool) (svpinst.cxx:188)
==32095==    by 0x170B2D25: SvpSalInstance::Yield(bool, bool) (svpinst.cxx:317)
==32095==    by 0x91460DF: ImplYield(bool, bool) (svapp.cxx:363)
==32095==    by 0x9142814: Application::Yield() (svapp.cxx:395)
==32095==    by 0x91427C2: Application::Execute() (svapp.cxx:344)
==32095==    by 0x4EC7589: desktop::Desktop::Main() (app.cxx:1680)
==32095==  Address 0x1d498b28 is 408 bytes inside a block of size 520 free'd
==32095==    at 0x4A078DE: operator delete(void*) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==32095==    by 0x954CFE5: VirtualDevice::~VirtualDevice() (virdev.cxx:202)
==32095==    by 0x1EE26F7D: SdModule::~SdModule() (sdmod.cxx:128)
==32095==    by 0x1EE26FFD: SdModule::~SdModule() (sdmod.cxx:129)
==32095==    by 0x65C9E47: SfxModule::DestroyModules_Impl() (module.cxx:327)
==32095==    by 0x654116A: SfxApplication::~SfxApplication() (app.cxx:226)
==32095==    by 0x6541249: SfxApplication::~SfxApplication() (app.cxx:244)
==32095==    by 0x655E452: SfxTerminateListener_Impl::notifyTermination(com::sun::star::lang::EventObject const&) (appinit.cxx:121)
==32095==    by 0x1B8A67A1: framework::Desktop::terminate() (desktop.cxx:343)

Change-Id: Ib70444c2b0fc60daa094323510d20b146a416087