layout: Implement basic lists and the CSS1 list properties.

The exact rendering is ill-spec'd. Some things are ugly (especially the
width and height of list style images) but they are infrequently used
and I believe this implementation matches the spec. Numeric lists are
not supported yet, since they will require a separate layout pass.

The implementation is a subclass of `BlockFlow`, on advice from Robert
O'Callahan.

components/layout/flow.rs

@@ -430,6 +430,7 @@ pub trait MutableOwnedFlowUtils {
 pub enum FlowClass {
     BlockFlowClass,
     InlineFlowClass,
+    ListItemFlowClass,
     TableWrapperFlowClass,
     TableFlowClass,
     TableColGroupFlowClass,