Fix: `ScrollStackControllerDelegate` methods calls

[NicFontana]

This PR adresses an issue that causes calls to the ScrollStackControllerDelegate methods

• scrollStackRowDidBecomeVisible(_:row:index:state:)
• scrollStackRowDidBecomeHidden(_:row:index:state:)

before the completion of the ScrollStack layout process, when each rows still don't have a valid frame.

This can happen for example when a ViewController uses a ScrollStack as a subview and inserts rows in a for-loop inside its viewDidLoad() method.

Now these methods are called:

1. Only once after all the rows have been added and the ScrollStack has been fully laid out
2. When the user scrolls

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A fix has been also made to the dispatchRowsVisibilityChangesTo(_:) ScrollStack method: it now takes into account new inserted rows which still don't have a previous visibility state.

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Finally, the call to askForCutomizedSizeOfContentView(animated:) of ScrollStackRow has been moved back in the updateConstraints() method following the best practices described in ModifyingConstraints:

To batch a change, instead of making the change directly, call the setNeedsUpdateConstraints method on the view holding the constraint. Then, override the view’s updateConstraints method to modify the affected constraints. Always call the superclasses implementation as the last step of your updateConstraints method’s implementation.

This is necessary because we call setNeedsUpdateConstraints() in the layoutUI() method of ScrollStackRow.