SessionManager: can create new sessions and manage categories. Also provide a facade to register new session handlers. Session manager provide sthe list of handlers by order of priority (categories order and categories priority) either for the startup or the shutdown,
WorkingSession: unique object for the current smalltalk session (I.e. between a startup and a shutdown). On each image startup the current session is invalidated and a new session is created. Define how errors should be handled. Run the startup / shutdown process.
SessionCategory: we defined some well known categories for startup / shutdown: system, network, GUI, tools and there is room to add user specific categories, It help to run the handler at the right time without caring about the whole order.
and SessionHandlers: handleSession for a specific class or any other object.

the shutdown list is now simply the startup list reversed
we also refactored error handling at startup by using the UIManager and StartupUIManager. It allowed to remove ugly checks like #isInteractive, isHeadless in error handling methods and centralize that in UIManager.