message: mavsdk: deprecate by-reference accessors
  We want to transition from the reference to a system to a
  vector of pointers to systems.
diff: @@ -235,9 +235,12 @@ public:
  *
  * This returns the first discovered system or a null system if no system has yet been found.
  *
  + * @note This method will be deprecated because it will be replaced by a
  + * vector of system pointers.
  + *
  * @return A reference to a system.
  */
  - System& system() const;
  + DEPRECATED System& system() const;
  /**
  * @brief Get the system with the specified UUID (deprecated).
  @@ -271,9 +274,12 @@ public:
  *
  * If multiple systems have connected, this will return `false`.
  *
  + * @note This method will be deprecated because is_connected will be a
  + * method of the system, not of mavsdk.
  + *
  * @return `true` if exactly one system is connected.
  */
  - bool is_connected() const;
  + DEPRECATED bool is_connected() const;
  /**
  * @brief Returns `true` if a system is currently connected (deprecated).
  @@ -300,10 +306,13 @@ public:
  * @note Only one callback can be registered at a time. If this function is called several
  * times, previous callbacks will be overwritten.
  *
  + * @note This method will be deprecated because event_callback_t is
  + * deprecated and it will be replaced by `register_on_change()`.
  + *
  * @param callback Callback to register.
  *
  */
  - void register_on_discover(event_callback_t callback);
  + DEPRECATED void register_on_discover(event_callback_t callback);
  /**
  * @brief Register callback for system timeout.
  @@ -314,9 +323,12 @@ public:
  * @note Only one callback can be registered at a time. If this function is called several
  * times, previous callbacks will be overwritten.
  *
  + * @note This method will be deprecated because event_callback_t is
  + * deprecated and it will be replaced by `register_on_change()`.
  + *
  * @param callback Callback to register.
  */
  - void register_on_timeout(event_callback_t callback);
  + DEPRECATED void register_on_timeout(event_callback_t callback);
  private:
  /* @private. */
  