Fix #112: Address a few inconsistencies and make code more idiomatic

include/sdbus-c++/AdaptorInterfaces.h

@@ -141,6 +141,7 @@ namespace sdbus {
 
     protected:
         using base_type = AdaptorInterfaces;
+        ~AdaptorInterfaces() = default;
     };
 
 }

include/sdbus-c++/Message.h

@@ -44,7 +44,7 @@ namespace sdbus {
     class ObjectPath;
     class Signature;
     template <typename... _ValueTypes> class Struct;
-    struct UnixFd;
+    class UnixFd;
     class MethodReply;
     namespace internal {
         class ISdBus;

include/sdbus-c++/ProxyInterfaces.h

@@ -175,6 +175,7 @@ namespace sdbus {
 
     protected:
         using base_type = ProxyInterfaces;
+        ~ProxyInterfaces() = default;
     };
 
 }

include/sdbus-c++/TypeTraits.h

@@ -41,7 +41,7 @@ namespace sdbus {
     template <typename... _ValueTypes> class Struct;
     class ObjectPath;
     class Signature;
-    struct UnixFd;
+    class UnixFd;
     class MethodCall;
     class MethodReply;
     class Signal;