[NUI] separate layouting logic of the independent children

[NUI] separate layouting logic of the independent children

This patch introduces `IterateLayoutChildren()` method that return
an enumerable collection of the child layouts that affected by parent
layout.

Independent children will be measured in `OnMeasureIndependentChildren()`.

By using `IterateLayoutChildren()`, we can remove the codes that check
whether child layout exclude from layout flow in `OnMeasure`. and api
users don't need to check `View.ExcludeLayouting` property in custom layouts.