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/Forward.h

@@ -53,6 +53,8 @@ class LoadRequest;
 class MouseEvent;
 class Node;
 class Origin;
+class Page;
+class PageClient;
 class Resource;
 class ResourceLoader;
 class Selector;