LibWeb: Add a naive Resource cache

This patch introduces a caching mechanism in ResourceLoader. It's keyed
on a LoadRequest object which is what you provide to load_resource()
when you want to load a resource.

We currently never prune the cache, so resources will stay in there
forever. This is obviously not gonna stay that way, but we're just
getting started here. :^)

This should drastically reduce the number of requests when loading
some sites (like Twitter) that reuse the same images over and over.

Libraries/LibWeb/Forward.h

@@ -48,6 +48,7 @@ class PageView;
 class ImageData;
 class LayoutDocument;
 class LayoutNode;
+class LoadRequest;
 class MouseEvent;
 class Node;
 class Origin;