LibWeb: Start building a new Resource class to share more resources

A Resource represents a resource that we're loading, have loaded or
will soon load. Basically, it's a downloadable resource that can be
shared by multiple clients.

A typical usecase is multiple <img> elements with the same src.
In a future patch, we will try to make sure that those <img> elements
get the same Resource if possible. This will reduce network usage,
memory usage, and CPU usage. :^)

For now, this first patch simply introduces the mechanism.

You get a Resource by calling ResourceLoader::load_resource().
To get notified about changes to a Resource's load status, you inherit
from ResourceClient and implement the callbacks you're interested in.

This patch turns HTMLImageElement into a ResourceClient.

Libraries/LibWeb/Loader/ResourceLoader.cpp

@@ -31,6 +31,7 @@
 #include <LibCore/File.h>
 #include <LibProtocol/Client.h>
 #include <LibProtocol/Download.h>
+#include <LibWeb/Loader/Resource.h>
 #include <LibWeb/Loader/ResourceLoader.h>
 
 namespace Web {
@@ -68,6 +69,25 @@ void ResourceLoader::load_sync(const URL& url, Function<void(const ByteBuffer&,
     loop.exec();
 }
 
+RefPtr<Resource> ResourceLoader::load_resource(const URL& url)
+{
+    if (!url.is_valid())
+        return nullptr;
+
+    auto resource = Resource::create({}, url);
+
+    load(
+        url,
+        [=](auto& data, auto& headers) {
+            const_cast<Resource&>(*resource).did_load({}, data, headers);
+        },
+        [=](auto& error) {
+            const_cast<Resource&>(*resource).did_fail({}, error);
+        });
+
+    return resource;
+}
+
 void ResourceLoader::load(const URL& url, Function<void(const ByteBuffer&, const HashMap<String, String, CaseInsensitiveStringTraits>& response_headers)> success_callback, Function<void(const String&)> error_callback)
 {
     if (is_port_blocked(url.port())) {