Add Shutdown Event

src/octoprint/events.py

@@ -26,6 +26,7 @@ def all_events():
 class Events(object):
 	# application startup
 	STARTUP = "Startup"
+	SHUTDOWN = "Shutdown"
 
 	# connect/disconnect to printer
 	CONNECTING = "Connecting"

src/octoprint/server/__init__.py

@@ -511,6 +511,7 @@ class Server():
 			self._logger.info("Shutting down...")
 			observer.stop()
 			observer.join()
+			eventManager.fire(events.Events.SHUTDOWN)
 			octoprint.plugin.call_plugin(octoprint.plugin.ShutdownPlugin,
 			                             "on_shutdown",
 			                             sorting_context="ShutdownPlugin.on_shutdown")