Add the ipc shutdown event (#2652)

This event is triggered when the connection to the ipc is about to
shutdown because of a user action such as with a `restart` or `exit`
command. The `change` field indicates why the ipc is shutting down. It
can be either "restart" or "exit".

fixes #2318

src/util.c

@@ -259,7 +259,7 @@ void i3_restart(bool forget_layout) {
 
     restore_geometry();
 
-    ipc_shutdown();
+    ipc_shutdown(SHUTDOWN_REASON_RESTART);
 
     LOG("restarting \"%s\"...\n", start_argv[0]);
     /* make sure -a is in the argument list or add it */