Fix TextBox in AdornerLayer causes collection modified exception in Arrange pass

[BAndysc]

What does the pull request do?

This PR changes VisualLayerManager so that layers can be added during its Arrange pass.

What is the current behavior?

In my previous PR #14484 I intentionally didn't touch ArrangeOverride method, as I didn't want to touch things that are not directly required.

However, it turned out it is required.

I assumed that Measure will always be called before Arrange. However, this is not always true. Consider this:

root.Measure(new Size(10, 10));

var adorner = new TextBox();
adornerLayer.Children.Add(adorner);
AdornerLayer.SetAdornedElement(adorner, button);

root.Arrange(new Rect(0, 0, 10, 10));

In root.Arrange, root already has valid measurement (IsMeasureValid = true) yet in the meantime a new control was added, which will reference VisualLayerManager.TextSelectorLayer -> crash.

What is the updated/expected behavior with this PR?

Both Measure and Arrange methods in VisualLayerManager will support adding layers during iteration.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

n/a

Obsoletions / Deprecations

n/a

Fixed issues

#14483

[TomEdwardsEnscape]

This pattern means that items in the collection can be skipped, or arranged twice. See #13498 for a more robust solution.

[BAndysc]

@TomEdwardsEnscape VisualLayerManager is a very specific case of a container, elements are only added to it, never removed or moved, so this is not a problem.

Also elements are added only to the end of the list, so not only this never arrange elements twice, but also never skips new elements.