LibHTML: Implement basic :hover pseudo-class support

This is currently very aggressive. Whenever the Document's hovered node
changes, we invalidate all style and do a full relayout.

It does look cool though. So cool that I'm adding it to the default
stylesheet. :^)

Libraries/LibHTML/CSS/Default.css

@@ -90,6 +90,10 @@ a {
     text-decoration: underline;
 }
 
+a:hover {
+    color: red;
+}
+
 hr {
     margin-top: 4;
     margin-bottom: 4;

Libraries/LibHTML/CSS/SelectorEngine.cpp

@@ -1,10 +1,33 @@
 #include <LibHTML/CSS/SelectorEngine.h>
+#include <LibHTML/DOM/Document.h>
 #include <LibHTML/DOM/Element.h>
 
 namespace SelectorEngine {
 
+static bool matches_hover_pseudo_class(const Element& element)
+{
+    auto* hovered_node = element.document().hovered_node();
+    if (!hovered_node)
+        return false;
+    if (&element == hovered_node)
+        return true;
+    return element.is_ancestor_of(*hovered_node);
+}
+
 bool matches(const Selector::Component& component, const Element& element)
 {
+    switch (component.pseudo_class) {
+    case Selector::Component::PseudoClass::None:
+        break;
+    case Selector::Component::PseudoClass::Link:
+        ASSERT_NOT_REACHED();
+        break;
+    case Selector::Component::PseudoClass::Hover:
+        if (!matches_hover_pseudo_class(element))
+            return false;
+        break;
+    }
+
     switch (component.type) {
     case Selector::Component::Type::Id:
         return component.value == element.attribute("id");

Libraries/LibHTML/DOM/Document.cpp

@@ -195,3 +195,13 @@ const LayoutDocument* Document::layout_node() const
 {
     return static_cast<const LayoutDocument*>(Node::layout_node());
 }
+
+void Document::set_hovered_node(Node* node)
+{
+    if (m_hovered_node == node)
+        return;
+
+    m_hovered_node = node;
+    invalidate_style();
+}
+

Libraries/LibHTML/DOM/Document.h

@@ -38,7 +38,7 @@ public:
 
     virtual String tag_name() const override { return "#document"; }
 
-    void set_hovered_node(Node* node) { m_hovered_node = node; }
+    void set_hovered_node(Node*);
     Node* hovered_node() { return m_hovered_node; }
     const Node* hovered_node() const { return m_hovered_node; }