refactor: invert dependency between Message and Connection (#457)

This reorganizes the layers of abstraction in the sense how `Message` depends on `Connection` and vice versa. Now, `Message` has a link to the `Connection`. This replaces the shortcut link to the low-level `SdBus` interface that the `Message` kept. The interactions from `Message` now go through `Connection` which forwards them to `SdBus`. `Connection` is now a sole owner of the low-level `SdBus` interface. This allows for future changes around `SdBus` (e.g. a change from virtual functions back to non-virtual functions) without affecting the rest of the library. `Proxy`s and `Object`s can now send messages directly without having to go through `Connection`. The `Connection` no more depends on `Message` business-logic methods; it serves only as a factory for messages.

The flow for creating messages: `Proxy`/`Object` -> `Connection` -> `SdBus`
The flow for sending messages: (`Proxy`/`Object` ->) `Message` -> `Connection` -> `SdBus`

This also better reflects how dependencies are managed in the underlying sd-bus library.

Additionally, `getSdBusInterface()` methods are removed which was anyway planned, and improves the design by "Tell, don't ask" principle.

This refactoring is the necessary enabler for other upcoming improvements (regarding sending long messages, or creds refactoring, for example).

include/sdbus-c++/IObject.h

@@ -403,7 +403,7 @@ namespace sdbus {
          *
          * @throws sdbus::Error in case of failure
          */
-        [[nodiscard]] virtual Signal createSignal(const InterfaceName& interfaceName, const SignalName& signalName) = 0;
+        [[nodiscard]] virtual Signal createSignal(const InterfaceName& interfaceName, const SignalName& signalName) const = 0;
 
         /*!
          * @brief Emits signal for this object path
@@ -419,7 +419,7 @@ namespace sdbus {
     protected: // Internal API for efficiency reasons used by high-level API helper classes
         friend SignalEmitter;
 
-        [[nodiscard]] virtual Signal createSignal(const char* interfaceName, const char* signalName) = 0;
+        [[nodiscard]] virtual Signal createSignal(const char* interfaceName, const char* signalName) const = 0;
     };
 
     // Out-of-line member definitions

include/sdbus-c++/IProxy.h

@@ -364,7 +364,7 @@ namespace sdbus {
          *
          * @throws sdbus::Error in case of failure
          */
-        [[nodiscard]] virtual MethodCall createMethodCall(const InterfaceName& interfaceName, const MethodName& methodName) = 0;
+        [[nodiscard]] virtual MethodCall createMethodCall(const InterfaceName& interfaceName, const MethodName& methodName) const = 0;
 
         /*!
          * @brief Calls method on the remote D-Bus object
@@ -652,7 +652,7 @@ namespace sdbus {
         friend AsyncMethodInvoker;
         friend SignalSubscriber;
 
-        [[nodiscard]] virtual MethodCall createMethodCall(const char* interfaceName, const char* methodName) = 0;
+        [[nodiscard]] virtual MethodCall createMethodCall(const char* interfaceName, const char* methodName) const = 0;
         virtual void registerSignalHandler( const char* interfaceName
                                           , const char* signalName
                                           , signal_handler signalHandler ) = 0;

include/sdbus-c++/Message.h

@@ -58,7 +58,7 @@ namespace sdbus {
     class UnixFd;
     class MethodReply;
     namespace internal {
-        class ISdBus;
+        class IConnection;
     }
 }
 
@@ -245,15 +245,15 @@ namespace sdbus {
 
     protected:
         Message() = default;
-        explicit Message(internal::ISdBus* sdbus) noexcept;
-        Message(void *msg, internal::ISdBus* sdbus) noexcept;
-        Message(void *msg, internal::ISdBus* sdbus, adopt_message_t) noexcept;
+        explicit Message(internal::IConnection* connection) noexcept;
+        Message(void *msg, internal::IConnection* connection) noexcept;
+        Message(void *msg, internal::IConnection* connection, adopt_message_t) noexcept;
 
         friend Factory;
 
     protected:
         void* msg_{};
-        internal::ISdBus* sdbus_{};
+        internal::IConnection* connection_{};
         mutable bool ok_{true};
     };
 
@@ -275,7 +275,7 @@ namespace sdbus {
         bool doesntExpectReply() const;
 
     protected:
-        MethodCall(void *msg, internal::ISdBus* sdbus, adopt_message_t) noexcept;
+        MethodCall(void *msg, internal::IConnection* connection, adopt_message_t) noexcept;
 
     private:
         MethodReply sendWithReply(uint64_t timeout = 0) const;