Specifying x::w::dim_axis_arg values

A x::w::dim_axis_arg value is specified as three discrete dim elements:

<height>
  <min>list-min</min>
  <preferred>list-preferred</preferred>
  <max>list-max</max>
</height>

<!-- ..... -->

<dim id="list-min">10</dim>
<dim id="list-preferred">100</dim>
<dim id="list-max">inf</dim>

Individual min, preferred, and max values set the minimum, preferred, and maximum values of a x::w::dim_axis_arg by specifying ids of other dim elements in the theme file.