Implement NetworkManager

For now there are two methods:

- `NETWORK:IsUrlAllowed()`: Check whether access to a certain URL is allowed.
- `NETWORK:HttpRequest()`: Perform an HTTP request.

By default access to the network is disabled for all target hosts. It
can be enabled by setting `HttpEnable=1` in the preferences. Individual
hosts have to be added to `HttpAllowHosts` as a comma separated list to
allow access.

See included docs for more details on usage.

extern/IXWebSocket-11.3.2/ixwebsocket/IXCancellationRequest.cpp

@@ -0,0 +1,35 @@
+/*
+ *  IXCancellationRequest.cpp
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2019 Machine Zone, Inc. All rights reserved.
+ */
+
+#include "IXCancellationRequest.h"
+
+#include <cassert>
+#include <chrono>
+
+namespace ix
+{
+    CancellationRequest makeCancellationRequestWithTimeout(
+        int secs, std::atomic<bool>& requestInitCancellation)
+    {
+        assert(secs > 0);
+
+        auto start = std::chrono::system_clock::now();
+        auto timeout = std::chrono::seconds(secs);
+
+        auto isCancellationRequested = [&requestInitCancellation, start, timeout]() -> bool {
+            // Was an explicit cancellation requested ?
+            if (requestInitCancellation) return true;
+
+            auto now = std::chrono::system_clock::now();
+            if ((now - start) > timeout) return true;
+
+            // No cancellation request
+            return false;
+        };
+
+        return isCancellationRequested;
+    }
+} // namespace ix