Sends a startup notification message of type
message_type to this.
This is a convenience function for use by code that implements the freedesktop startup notification specification. Applications should not normally need to call it directly. See the Startup Notification Protocol specification for definitions of the message types and keys that can be used.
startup notification message type ("new", "change", or "remove")
a list of key/value pairs (as strings), terminated by a null key. (A null value for a key will cause that key to be skipped in the output.)