Sets the startup notification ID for a display.
This is usually taken from the value of the DESKTOP_STARTUP_ID environment variable, but in some cases (such as the application not being
exec) it can come from other sources.
If the ID contains the string "_TIME" then the portion following that string is taken to be the X11 timestamp of the event that triggered the application to be launched and the GDK current event time is set accordingly.
The startup ID is also what is used to signal that the startup is complete (for example, when opening a window or when calling notify_startup_complete).
the startup notification ID (must be valid utf8)