some refactoring with optional_function

- wrapped std::function into utils::optional_function
-- cleaned the code accordingly in windows_emulator
- using the 'emulator'/'windows_emulator' dependency implies the emulator_common as well.

src/common/utils/function.hpp

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <functional>
+
+namespace utils
+{
+    template <typename Signature>
+    class optional_function;
+
+    template <typename Ret, typename... Args>
+    class optional_function<Ret(Args...)>
+    {
+      private:
+        std::function<Ret(Args...)> func;
+
+      public:
+        optional_function() = default;
+        optional_function(std::function<Ret(Args...)> f)
+            : func(std::move(f))
+        {
+        }
+
+        optional_function& operator=(std::function<Ret(Args...)> f)
+        {
+            func = std::move(f);
+            return *this;
+        }
+
+        Ret operator()(Args... args) const
+        {
+            if (func)
+            {
+                return func(std::forward<Args>(args)...);
+            }
+            else
+            {
+                if constexpr (!std::is_void_v<Ret>)
+                {
+                    return Ret();
+                }
+            }
+        }
+        explicit operator bool() const
+        {
+            return static_cast<bool>(func);
+        }
+    };
+}