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/LayoutImage.cpp

@@ -0,0 +1,20 @@
+#include <LibHTML/Layout/LayoutImage.h>
+
+LayoutImage::LayoutImage(const HTMLImageElement& element, NonnullRefPtr<StyleProperties> style)
+    : LayoutReplaced(element, move(style))
+{
+}
+
+LayoutImage::~LayoutImage()
+{
+}
+
+void LayoutImage::layout()
+{
+    LayoutReplaced::layout();
+}
+
+void LayoutImage::render(RenderingContext& context)
+{
+    LayoutReplaced::render(context);
+}