BPMN21 — Add start and intermediate events of type "User"

Introduce start and intermediate events of type “User”. Events of type User can be used to identify user initiated actions for example pressing a door-bell or selecting a menu item on a computer. At the moment the BPMN specification only allows these events to be modelled as conditional events or alternatively through the BPMN extension mechanism. Unfortunately the majority of vendors do not support the extension mechanism, they only support the main specification. Adding this to the main specification would greatly improve the BPMN's applicability and usability.