overlay: OBS control window via WebSocket API (#773)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds an overlay window widget for controlling OBS over WebSocket.

Add new lines to the FPS widget that shows recording / streaming timers.

Show notifications for major state changes (stream started/stopped,
recording started/paused/resumed/stopped)

## Testing

src/spice2x/external/easywsclient/easywsclient.hpp

@@ -0,0 +1,73 @@
+#ifndef EASYWSCLIENT_HPP_20120819_MIOFVASDTNUASZDQPLFD
+#define EASYWSCLIENT_HPP_20120819_MIOFVASDTNUASZDQPLFD
+
+// This code comes from:
+// https://github.com/dhbaird/easywsclient
+//
+// To get the latest version:
+// wget https://raw.github.com/dhbaird/easywsclient/master/easywsclient.hpp
+// wget https://raw.github.com/dhbaird/easywsclient/master/easywsclient.cpp
+
+#include <string>
+#include <vector>
+#include <cstdint>
+
+namespace easywsclient {
+
+struct Callback_Imp { virtual void operator()(const std::string& message) = 0; };
+struct BytesCallback_Imp { virtual void operator()(const std::vector<uint8_t>& message) = 0; };
+
+class WebSocket {
+  public:
+    typedef WebSocket * pointer;
+    typedef enum readyStateValues { CLOSING, CLOSED, CONNECTING, OPEN } readyStateValues;
+
+    // Factories:
+    static pointer create_dummy();
+    static pointer from_url(const std::string& url, const std::string& origin = std::string());
+    static pointer from_url_no_mask(const std::string& url, const std::string& origin = std::string());
+
+    // Interfaces:
+    virtual ~WebSocket() { }
+    virtual void poll(int timeout = 0) = 0; // timeout in milliseconds
+    virtual void send(const std::string& message) = 0;
+    virtual void sendBinary(const std::string& message) = 0;
+    virtual void sendBinary(const std::vector<uint8_t>& message) = 0;
+    virtual void sendPing() = 0;
+    virtual void close() = 0;
+    virtual readyStateValues getReadyState() const = 0;
+
+    template<class Callable>
+    void dispatch(Callable callable)
+        // For callbacks that accept a string argument.
+    { // N.B. this is compatible with both C++11 lambdas, functors and C function pointers
+        struct _Callback : public Callback_Imp {
+            Callable& callable;
+            _Callback(Callable& callable) : callable(callable) { }
+            void operator()(const std::string& message) { callable(message); }
+        };
+        _Callback callback(callable);
+        _dispatch(callback);
+    }
+
+    template<class Callable>
+    void dispatchBinary(Callable callable)
+        // For callbacks that accept a std::vector<uint8_t> argument.
+    { // N.B. this is compatible with both C++11 lambdas, functors and C function pointers
+        struct _Callback : public BytesCallback_Imp {
+            Callable& callable;
+            _Callback(Callable& callable) : callable(callable) { }
+            void operator()(const std::vector<uint8_t>& message) { callable(message); }
+        };
+        _Callback callback(callable);
+        _dispatchBinary(callback);
+    }
+
+  protected:
+    virtual void _dispatch(Callback_Imp& callable) = 0;
+    virtual void _dispatchBinary(BytesCallback_Imp& callable) = 0;
+};
+
+} // namespace easywsclient
+
+#endif /* EASYWSCLIENT_HPP_20120819_MIOFVASDTNUASZDQPLFD */