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/DOM/HTMLImageElement.cpp

@@ -0,0 +1,10 @@
+#include <LibHTML/DOM/HTMLImageElement.h>
+
+HTMLImageElement::HTMLImageElement(Document& document, const String& tag_name)
+    : HTMLElement(document, tag_name)
+{
+}
+
+HTMLImageElement::~HTMLImageElement()
+{
+}