appDisplay: Refresh folder preview when favorites change

appDisplay: Refresh folder preview when favorites change

Pinning an app from inside a folder did not refresh the
folder preview. FolderIcon._sync() was only listening
to folder GSettings changes, while pinning updates the
favorites setting instead.

As a result, _sync() was never called and the preview
could show stale icons or remain visible when the folder
became empty.

Fix this by also connecting _sync() to
AppFavorites::changed, ensuring the folder preview
updates correctly when favorites are modified.

Closes : https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/9026

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4088>
(cherry picked from commit e9f24200e22b7a2e311224c4f16f7f0488ce6a11)