Determines when the flap will fold.
If set to `ADW_FOLD_THRESHOLD_POLICY_MINIMUM`, flap will only fold when the children cannot fit anymore. With `ADW_FOLD_THRESHOLD_POLICY_NATURAL`, it will fold as soon as children don't get their natural size.
This can be useful if you have a long ellipsizing label and want to let it ellipsize instead of immediately folding.