Redesign inheritance from Message (#62)

... so that the code is more idiomatic, clear and expressive about its intended use

src/MessageUtils.h

@@ -30,7 +30,35 @@
 
 namespace sdbus
 {
-    Message createPlainMessage();
+    class Message::Factory
+    {
+    public:
+        template<typename _Msg>
+        static _Msg create()
+        {
+            return _Msg{};
+        }
+
+        template<typename _Msg>
+        static _Msg create(void *msg)
+        {
+            return _Msg{msg};
+        }
+
+        template<typename _Msg>
+        static _Msg create(void *msg, internal::ISdBus* sdbus)
+        {
+            return _Msg{msg, sdbus};
+        }
+
+        template<typename _Msg>
+        static _Msg create(void *msg, internal::ISdBus* sdbus, adopt_message_t)
+        {
+            return _Msg{msg, sdbus, adopt_message};
+        }
+    };
+
+    PlainMessage createPlainMessage();
 }
 
 #endif /* SDBUS_CXX_INTERNAL_MESSAGEUTILS_H_ */