appDisplay: Avoid a warning
At least the last stable release of gjs has an issue with async handlers for custom (i.e. defined in JS) GObject signals.
The handler still works, but evoking it prints the following warning:
JS ERROR: Error: Could not guess unspecified GValue type
We can avoid the warning by using the addAction() convenience method, which makes for a small nice cleanup anyway.
Fixes #4531 (closed)