LibHTML: Batch style updates and make them start from the root

Use a zero-timer to schedule a style update after invalidating style
on any node. Nodes now have a needs_style_update flag which helps us
batch and coalesce the work.

We also start style updates at the root and work our way through the
document, updating any node that has the needs_style_update flag set.
This is slower than what we were doing before, but far more correct.

There is a ton of room for improvement here. :^)

Libraries/LibHTML/DOM/Node.h

@@ -70,6 +70,9 @@ public:
 
     virtual bool is_child_allowed(const Node&) const { return true; }
 
+    bool needs_style_update() const { return m_needs_style_update; }
+    void set_needs_style_update(bool value) { m_needs_style_update = value; }
+
     void invalidate_style();
 
 protected:
@@ -78,6 +81,7 @@ protected:
     Document& m_document;
     mutable LayoutNode* m_layout_node { nullptr };
     NodeType m_type { NodeType::INVALID };
+    bool m_needs_style_update { false };
 };
 
 template<typename T>