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 launched using exec) it can come from other sources.
The startup ID is also what is used to signal that the startup is complete (for example, when opening a window or when calling [ [email protected]_startup_complete]).
| this |
a `GdkDisplay` |
| startup_id |
the startup notification ID (must be valid utf8) |