dash: Fixes wrong separator position

Sometimes when an icon is dragged and dropped in Dash, one Favourite app icon
can appear to the right of Dash separator. This can happen when available system
resources are low and the PlaceHolder destroy animation is delayed and the
corresponding container child is still present. With this commit the separator
position is calculated correctly, even when the animation is pending.

Fixes #3966, #3799

(cherry picked from commit 07579f05)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1982>