LibWeb: Add Page abstraction between PageView and main Frame

* A PageView is a view onto a Page object.
* A Page always has a main Frame (root of Frame tree.)
* Page has a PageClient. PageView is a PageClient.

The goal here is to allow building another kind of view onto
a Page while keeping the rest of LibWeb intact.

Libraries/LibWeb/CMakeLists.txt

@@ -96,6 +96,7 @@ set(SOURCES
     Loader/ImageResource.cpp
     Loader/Resource.cpp
     Loader/ResourceLoader.cpp
+    Page.cpp
     PageView.cpp
     Parser/CSSParser.cpp
     Parser/Entities.cpp