LibHTML: Flesh out <img> element with LayoutImage and LayoutReplaced

This patch adds parsing of <img> into HTMLImageElement objects.
It also adds LayoutImage and its parent class LayoutReplaced, which is
going to represent CSS "replaced elements."

Libraries/LibHTML/Layout/LayoutReplaced.h

@@ -0,0 +1,12 @@
+#include <LibHTML/DOM/Element.h>
+#include <LibHTML/Layout/LayoutNode.h>
+
+class LayoutReplaced : public LayoutNode {
+public:
+    LayoutReplaced(const Element&, NonnullRefPtr<StyleProperties>);
+    virtual ~LayoutReplaced();
+
+    const Element& node() const { return static_cast<const Element&>(*LayoutNode::node()); }
+
+private:
+};