Most uses of builtin icons (check and radio buttons,
expanders, etc) are placed next to labels, so they should
be properly positioned wrt to the baseline. Lacking anything
better, give the builtin icons a baseline that places the
center of the icon at the strikethrough position.
The gtk_builtin_icon_get_default_size_property returns a const char *,
in a way such that some compilers insist that something that is of a
pointer value be returned, so fix that by replacing 0 with NULL.
Make min-width/height have preference over the set default size. This
allows shrinking the widget. The default size is only used if min-width
is not set (or explicitly set to 0.
