LibHTML: Implement the <blink> element

Just in time for Serenity's 1st birthday, here is the <blink> element!

This patch adds a bunch of different mechanisms to enable partial
repaints of the layout tree (LayoutNode::set_needs_display()))
It also adds LayoutNode::is_visible(), which can be toggled to prevent
a LayoutNode from rendering anything (it still takes up space though.)

Libraries/LibHTML/DOM/HTMLBlinkElement.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <LibHTML/DOM/HTMLElement.h>
+
+class CTimer;
+
+class HTMLBlinkElement : public HTMLElement {
+public:
+    HTMLBlinkElement(Document&, const String& tag_name);
+    virtual ~HTMLBlinkElement() override;
+
+private:
+    void blink();
+
+    NonnullRefPtr<CTimer> m_timer;
+};