Interface holding the size and alignment lines of the measured layout, as well as the
children positioning logic.
placeChildren is the function used for positioning children. Placeable.placeAt should
be called on children inside placeChildren.
The alignment lines can be used by the parent layouts to decide layout, and can be queried
using the Placeable.get operator. Note that alignment lines will be inherited by parent
layouts, such that indirect parents will be able to query them as well.
abstract fun placeChildren(): Unit
abstract val height: Int
abstract val width: Int