appDisplay: Avoid a warning

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.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4531

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1938>