Implement #104: add getObjectPath() for classes (#105)

* Implement #104: add getObjectPath() for classes

* Implement #104: changes requested in review

Co-authored-by: Christian Schneider <cschneider@radiodata.biz>

src/Proxy.h

@@ -60,6 +60,8 @@ namespace sdbus::internal {
         void finishRegistration() override;
         void unregister() override;
 
+        const std::string& getObjectPath() const override;
+
     private:
         class SyncCallReplyData
         {