Fix unused variable warnings
* src/eventsink.h (USE_INVOKE_ARGS): Also add dispid. * src/gpgoladdin.cpp (GpgolAddinFactory::CreateInstance): Unused punk. (GpgolAddin::Invoke, GpgolRibbonExtender::Invoke): Use invoke args. * src/mailitem.cpp (GpgolItemEvents::Write): Remove want_html var. * src/ribbon-callbacks.cpp (message_flag_status): Remove window param. (mark_mime_action): Update call accordingly.