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.cpp

@@ -0,0 +1,27 @@
+#include <LibCore/CTimer.h>
+#include <LibHTML/CSS/StyleProperties.h>
+#include <LibHTML/CSS/StyleValue.h>
+#include <LibHTML/DOM/HTMLBlinkElement.h>
+#include <LibHTML/Layout/LayoutNode.h>
+
+HTMLBlinkElement::HTMLBlinkElement(Document& document, const String& tag_name)
+    : HTMLElement(document, tag_name)
+    , m_timer(CTimer::construct())
+{
+    m_timer->set_interval(500);
+    m_timer->on_timeout = [this] { blink(); };
+    m_timer->start();
+}
+
+HTMLBlinkElement::~HTMLBlinkElement()
+{
+}
+
+void HTMLBlinkElement::blink()
+{
+    if (!layout_node())
+        return;
+
+    layout_node()->set_visible(!layout_node()->is_visible());
+    layout_node()->set_needs_display();
+}

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;
+};