The .popup-selection and .selection list models are currently
identical, because they take the same filter list model. This
means that, when filtering (with search) the selection may
change without user action.

Make the main .selection list model ignore the filter and base
itself directly on the main model. And reset search filter
whenever we need .popup-selection and .selection indexes to
match.

This logic is copied from GtkDropDown, to match its behavior.

Fixes https://gitlab.gnome.org/GNOME/libadwaita/-/issues/770