It is possible to set a (multi-)chooser's `selected-item(s)` to a non-available value

[nicolaspayette]

This is non-trivial to prevent, as no property validation is currently done before writing to the property map.

Perhaps it would be sufficient for the moment to make sure that no misleading visual feedback is provided in the widgets themselves. E.g., when doing:

xw:create-chooser "chooser" [
  xw:set-items ["a" "b" "c"]
  xw:set-selected-item "d"
]

The chooser still shows "c", even though xw:get "chooser" would report "d".