ProtocolServer+LibProtocol: Introduce a server for handling downloads

This patch adds ProtocolServer, a server that handles network requests
on behalf of its clients. The first protocol implemented is HTTP.

The idea here is to use a plug-in architecture where any number of
protocols can be added and implemented without having to mess around
with each client program that wants to use the protocol.

A simple client API is provided through LibProtocol::Client. :^)

Libraries/LibProtocol/Client.cpp

@@ -0,0 +1,45 @@
+#include <LibProtocol/Client.h>
+#include <SharedBuffer.h>
+
+namespace LibProtocol {
+
+Client::Client()
+    : ConnectionNG(*this, "/tmp/psportal")
+{
+}
+
+void Client::handshake()
+{
+    auto response = send_sync<ProtocolServer::Greet>(getpid());
+    set_server_pid(response->server_pid());
+    set_my_client_id(response->client_id());
+}
+
+bool Client::is_supported_protocol(const String& protocol)
+{
+    return send_sync<ProtocolServer::IsSupportedProtocol>(protocol)->supported();
+}
+
+i32 Client::start_download(const String& url)
+{
+    return send_sync<ProtocolServer::StartDownload>(url)->download_id();
+}
+
+bool Client::stop_download(i32 download_id)
+{
+    return send_sync<ProtocolServer::StopDownload>(download_id)->success();
+}
+
+void Client::handle(const ProtocolClient::DownloadFinished& message)
+{
+    if (on_download_finish)
+        on_download_finish(message.download_id(), message.success());
+}
+
+void Client::handle(const ProtocolClient::DownloadProgress& message)
+{
+    if (on_download_progress)
+        on_download_progress(message.download_id(), message.total_size(), message.downloaded_size());
+}
+
+}