The FlowLayout fails to provide a preferred size in case no sizing is
specified on one axis. It should, instead, have the preferred size of
the sum of its children, depending on the orientation property.
http://bugzilla.openedhand.com/show_bug.cgi?id=2013
FlowLayout should compute the correct height for the assigned width when
in horizontal flow, and the correct width for the assigned height when
in vertical flow. This means pre-computing the number of lines inside
the get_preferred_width() and get_preferred_height(). We can then cache
the computed column width and row height, cache them inside the layout
and then use them when allocating the children.
