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.cpp

@@ -0,0 +1,13 @@
+#include <LibHTML/DOM/Element.h>
+#include <LibHTML/Layout/LayoutReplaced.h>
+
+LayoutReplaced::LayoutReplaced(const Element& element, NonnullRefPtr<StyleProperties> style)
+    : LayoutNode(&element, move(style))
+{
+    // FIXME: Allow non-inline replaced elements.
+    set_inline(true);
+}
+
+LayoutReplaced::~LayoutReplaced()
+{
+}