Add tests for !LayoutContainer's new API: !LayoutContainer.design and children using region and layoutPriority.  Note that for backwards compatibility,  "design" shouldn't affect children that set layoutAlign instead of region.  This is basically "accidentally" working due to the way _getOrderedChildren() happens to be written.  Refs #16770 !strict. 

Also convert uses of deprecated layoutAlign in calls to utils.layoutChildren() to region, refs #16585 !strict.