LibWeb: Start adding support for the <iframe> element! :^)

This patch introduces a bunch of things:

- Subframes (Web::Frame::create_subframe())
- HTMLIFrameElement (loads and owns the hosted Web::Frame)
- LayoutFrame (layout and rendering of the hosted frame)

There's still a huge number of things missing, like scrolling, overflow
handling, event handling, scripting, etc. But we can make a little
iframe in a document and it actually renders another document there.
I think that's pretty cool! :^)

Libraries/LibWeb/Layout/LayoutReplaced.h

@@ -37,6 +37,7 @@ public:
     virtual ~LayoutReplaced() override;
 
     const Element& node() const { return to<Element>(*LayoutNode::node()); }
+    Element& node() { return to<Element>(*LayoutNode::node()); }
 
     virtual bool is_replaced() const final { return true; }