LibHTML: Add some convenient geometry getters on LayoutNode

Add x(), y(), size() and position() and use them around the codebase.

Libraries/LibHTML/Layout/LayoutListItem.cpp

@@ -21,6 +21,6 @@ void LayoutListItem::layout()
         append_child(*m_marker);
     }
 
-    Rect marker_rect { rect().x() - 8, rect().y(), 4, rect().height() };
+    Rect marker_rect { x() - 8, y(), 4, height() };
     m_marker->set_rect(marker_rect);
 }