LibHTML: Implement some very simple selector matching.

We walk the entire DOM and check all selectors against all elements. Only
id, class and tag name are checked right now. There's no ancestor stack
or compound selectors. All in good time :^)

LibHTML/DOM/Element.cpp

@@ -50,6 +50,19 @@ void Element::set_attributes(Vector<Attribute>&& attributes)
     m_attributes = move(attributes);
 }
 
+bool Element::has_class(const StringView& class_name) const
+{
+    auto value = attribute("class");
+    if (value.is_empty())
+        return false;
+    auto parts = value.split_view(' ');
+    for (auto& part : parts) {
+        if (part == class_name)
+            return true;
+    }
+    return false;
+}
+
 RefPtr<LayoutNode> Element::create_layout_node()
 {
     if (m_tag_name == "html")