refactor: invert dependency between Message and Connection

[sangelovic]

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. Proxys and Objects 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).