Start working on a Downloader app and backing classes in LibGUI.

LibGUI is slowly becoming LibKitchensink but I'm okay with this for now.

LibGUI/GNetworkResponse.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <AK/Retainable.h>
+#include <AK/ByteBuffer.h>
+
+class GNetworkResponse : public Retainable<GNetworkResponse> {
+public:
+    virtual ~GNetworkResponse();
+
+    bool is_error() const { return m_error; }
+    const ByteBuffer& payload() const { return m_payload; }
+
+protected:
+    explicit GNetworkResponse(ByteBuffer&&);
+
+    bool m_error { false };
+    ByteBuffer m_payload;
+};