LibHTML: Make "children are inline" flag imperative

Instead of computing whether a block's children are inline based on the
first child, make it an imperatively-set flag.

This gives us some flexibility to ignore things like text nodes inside
a <table>, for example. I'm still unsure what the "correct" way to deal
with those will be. We'll find out sooner or later. :^)

Libraries/LibHTML/Layout/LayoutTreeBuilder.cpp

@@ -42,6 +42,9 @@ static RefPtr<LayoutNode> create_layout_tree(Node& node, const StyleProperties*
         }
     }
 
+    if (have_inline_children && !have_block_children)
+        layout_node->set_children_are_inline(true);
+
     return layout_node;
 }