Add 'event.system.startup_done/shutdown' commands/events

[chros73]

A. Add event.system.* commands:

• event.system.startup_done : triggered when startup is done
  • e.g. after loading session files
  • it doesn't mean that processing all the downloads are finished at this point (e.g. event.download.resumed starts after this event)
• event.system.shutdown : triggered when shutdown is started

B. Usage examples:

# COMMAND/EVENT: Return startup time (can be used to calculate uptime)
method.insert  = system.startup_time, value, 0
method.set_key = event.system.startup_done, !!set_startup_time, {(branch, ((not,((system.startup_time)))), ((system.startup_time.set, (system.time))) )}
# Display startup time at startup
method.set_key = event.system.startup_done, ~log,  ((print, "Startup time: ", ((system.startup_time)) ))
# Prevent command to fire up on 'event.view.*' event during startup
method.set_key = event.view.show, ~print_name,  {(branch, ((system.startup_time)), ((print, ((ui.current_view)))) )}

# COMMAND/EVENT: Return shutdown time
method.insert  = system.shutdown_time, value|private, 0
method.set_key = event.system.shutdown, !!set_shutdown_time, {(branch, ((not,((system.shutdown_time)))), ((system.shutdown_time.set, (system.time))) )}
# Display shutdown time at shutdown
method.set_key = event.system.shutdown, ~log, ((print, "Shutdown time: ", ((system.shutdown_time)) ))
# Prevent command to fire up on 'event.download.paused/closed' event during shutdown
method.set_key = event.download.paused, ~print_name,  {(branch, ((not,((system.shutdown_time)))), ((print, ((d.name)))) )}