Create specific container for single-child usecase

[emilsjolander]

Row and Column are great APIs as they make the direction of the container explicit. However this causes some confusion when wanting a container with only a single child as both Row and Column will work. Therefor we would like to introduce some Container.Builder type which only accepts one child (possibly only 1 relative positioned child and however many absolute children).

I can think of two possible APIs:

SingleChildContainer.create(c)
   .child(...)
   .child(...) // throws exception
   ...;

or

MyComponent.create(c)
   .prop1(...)
   .withContainer()
   ....;

Point of contact: @emilsjolander