GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
The :wrap property is not implemented, and mostly useless: the
FlowLayout is a reflowing grid. This means that if it receives
less than the preferred width or height in the flow direction
then it should always reflow.
FlowLayout is a layout manager that arranges its children in a
reflowing line; the orientation controls the major axis for the
layout: horizontal, for reflow on the Y axis, and vertical, for
reflow on the X axis.
