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/Frame.cpp

@@ -32,6 +32,10 @@
 
 namespace Web {
 
+Frame::Frame()
+{
+}
+
 Frame::Frame(PageView& page_view)
     : m_page_view(page_view.make_weak_ptr())
 {