appDisplay: Reduce folder dialog popdown timeout

1500ms is too long of a delay, and it confuses people more than it
helps with accidental drops.

Reduce the timeout to 500ms as per design feedback.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3092

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1447