appDisplay: Properly destroy SwipeTracker on destroy

SwipeTracker connects to signals of the stage, but doesn't disconnect on
destroy, leaving them hanging and potentially running callbacks for
destroyed objects.

This is visible when removing a folder by dragging all icons out, and
running the swipe gestures, which will produce a bunch of warnings.

Explicitly destroy, remove, and disconnect the swipe tracker.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1518>

(cherry picked from commit 853644d7)