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/IXSelectInterruptFactory.cpp

@@ -0,0 +1,26 @@
+/*
+ *  IXSelectInterruptFactory.cpp
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2019 Machine Zone, Inc. All rights reserved.
+ */
+
+#include "IXSelectInterruptFactory.h"
+
+#include "IXUniquePtr.h"
+#if defined(__linux__) || defined(__APPLE__)
+#include "IXSelectInterruptPipe.h"
+#else
+#include "IXSelectInterrupt.h"
+#endif
+
+namespace ix
+{
+    SelectInterruptPtr createSelectInterrupt()
+    {
+#if defined(__linux__) || defined(__APPLE__)
+        return ix::make_unique<SelectInterruptPipe>();
+#else
+        return ix::make_unique<SelectInterrupt>();
+#endif
+    }
+} // namespace ix