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/CMakeLists.txt

@@ -48,6 +48,7 @@ set(SOURCES
     DOM/HTMLHeadingElement.cpp
     DOM/HTMLHRElement.cpp
     DOM/HTMLHtmlElement.cpp
+    DOM/HTMLIFrameElement.cpp
     DOM/HTMLImageElement.cpp
     DOM/HTMLInputElement.cpp
     DOM/HTMLLinkElement.cpp
@@ -70,6 +71,7 @@ set(SOURCES
     Layout/LayoutBreak.cpp
     Layout/LayoutCanvas.cpp
     Layout/LayoutDocument.cpp
+    Layout/LayoutFrame.cpp
     Layout/LayoutImage.cpp
     Layout/LayoutInline.cpp
     Layout/LayoutListItem.cpp