Expose `View::default_compute_layout`

[timboudreau]

While this method is implementable in custom component by copy-paste programming, it would be preferable for custom components to be able to keep up with any changes in the default implementation by calling it directly instead.

[dzhou121]

maybe we can put the method under ComputeLayoutCx and call it compute_view_children_layout?

[timboudreau]

Sure, sounds good.