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/GNetworkJob.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <LibGUI/GObject.h>
+#include <AK/Function.h>
+
+class GNetworkResponse;
+
+class GNetworkJob : public GObject {
+public:
+    enum class Error {
+        None,
+        ConnectionFailed,
+        TransmissionFailed,
+    };
+    virtual ~GNetworkJob() override;
+
+    Function<void(bool success)> on_finish;
+
+    bool has_error() const { return m_error != Error::None; }
+    Error error() const { return m_error; }
+    GNetworkResponse* response() { return m_response.ptr(); }
+    const GNetworkResponse* response() const { return m_response.ptr(); }
+
+    virtual void start() = 0;
+
+    virtual const char* class_name() const override { return "GNetworkJob"; }
+
+protected:
+    GNetworkJob();
+    void did_finish(Retained<GNetworkResponse>&&);
+    void did_fail(Error);
+
+private:
+    RetainPtr<GNetworkResponse> m_response;
+    Error m_error { Error::None };
+};