LibProtocol: Add a Download object so users don't have to manage ID's

LibProtocol::Client::start_download() now gives you a Download object
with convenient hooks (on_finish & on_progress).

Also, the IPC handshake is snuck into the Client constructor, so you
don't need to perform it after instantiating a Client.

This makes using LibProtocol much more pleasant. :^)

Libraries/LibProtocol/Download.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/ByteBuffer.h>
+#include <AK/Function.h>
+#include <AK/RefCounted.h>
+#include <AK/WeakPtr.h>
+
+class SharedBuffer;
+
+namespace LibProtocol {
+
+class Client;
+
+class Download : public RefCounted<Download> {
+public:
+    static NonnullRefPtr<Download> create_from_id(Badge<Client>, Client& client, i32 download_id)
+    {
+        return adopt(*new Download(client, download_id));
+    }
+
+    int id() const { return m_download_id; }
+    bool stop();
+
+    Function<void(bool success, const ByteBuffer& payload, RefPtr<SharedBuffer> payload_storage)> on_finish;
+    Function<void(u32 total_size, u32 downloaded_size)> on_progress;
+
+    void did_finish(Badge<Client>, bool success, u32 total_size, i32 shared_buffer_id);
+    void did_progress(Badge<Client>, u32 total_size, u32 downloaded_size);
+
+private:
+    explicit Download(Client&, i32 download_id);
+    WeakPtr<Client> m_client;
+    int m_download_id { -1 };
+};
+
+}