We need to add the row-spacing value when calculating the y position for lines
of actors in horizontal flowing layouts.
Similarly we need to add the col-spacing value when calculating the x posution
for actors in vertical flowing layouts.
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.
When changing the orientation of a FlowLayout, the associated
container should also change its request mode. A horizontally
flowing layout has a height depending on the width, since it
will reflow vertically; similarly, a vertically reflowing layout
will have a width depending on the height.
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.
Use the column and row size to align each child; with :homogeneous
set to TRUE, or with children with the same size, the FlowLayout
will behave like a reflowing grid.
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.
