runtime: Add a Signal class

Not very functional yet

api/sixtyfps-cpp/include/sixtyfps_signals.h

@@ -0,0 +1,33 @@
+#pragma once
+#include <string_view>
+#include "sixtyfps_signals_internal.h"
+
+namespace sixtyfps {
+
+// template<typename... Args>
+struct Signal
+{
+    Signal() { internal::sixtyfps_signal_init(&inner); }
+    ~Signal() { internal::sixtyfps_signal_drop(&inner); }
+    Signal(const Signal &) = delete;
+    Signal(Signal &&) = delete;
+    Signal &operator=(const Signal &) = delete;
+
+    template<typename F>
+    void set_handler(F binding)
+    {
+        internal::sixtyfps_signal_set_handler(
+                &inner,
+                [](const void *user_data, const void *value) {
+                    (*reinterpret_cast<F *>(user_data))(value);
+                },
+                new F(binding),
+                [](const void *user_data) { delete reinterpret_cast<F *>(user_data); });
+    }
+
+    void emit(void *data) const { internal::sixtyfps_signal_emit(&inner, data); }
+
+private:
+    internal::SignalOpaque inner;
+};
+}