Implement text alignment inside its content box for a text node using box-model.

[zwcloud]

For metrics's box-model inspector, this is temporarily implemented via reusing LayoutGroup in Metrics.LayoutTextInRectCentered:

ImGui/src/ImGui/Development/Metrics.cs

Lines 462 to 480 in 7d6f3be

	private static Rect LayoutTextInRectCentered(Rect rect, string text)
	{
	Node group = new Node(rect.GetHashCode());
	group.AttachLayoutGroup(false);
	group.ContentSize = rect.Size;
	group.RuleSet.ApplyOptions(GUILayout.Width(rect.Width).Height(rect.Height));
	group.RuleSet.AlignmentVertical = Alignment.Center;
	group.RuleSet.AlignmentHorizontal = Alignment.Center;
	var textNode = new Node(text.GetHashCode());
	textNode.AttachLayoutEntry();
	textNode.ContentSize = centeredLabelRuleSet.CalcSize(text);
	textNode.RuleSet.AlignmentVertical = Alignment.Center;
	textNode.RuleSet.AlignmentHorizontal = Alignment.Center;
	group.AppendChild(textNode);
	group.Layout(rect.TopLeft);
	return textNode.Rect;
	}

This should be somehow inlined to be used elsewhere.