Provide a function for measuring the baseline (in Y) of the contents of
the node. The layout engine uses this when the alignment is set to
baseline. Only works on leaf nodes (nodes with no further child layout
nodes).
Provide a function for measuring the desired dimensions of the LayoutNode.
This lets the layout engine know the intrinsic size of the contents. This
doesn't guarantee the final size of the node will match but lets the
layout attempt to provide the desired size when possible. Only works on
leaf nodes (nodes with no further child layout nodes).
