Port DBus pending calls to QCoro

so, this is how it would've looked like if we used coroutines instead of connecting continuations as signal handlers.

Pros: control flow seems linear.

Cons: Gotta manually guard QObjects with weak QPointers or strong QSharedPointer (even though in Qt paradigm we don't use them much).

On a bright side, unlike single-target QObject::connect, we can control have many and which objects to retain or guard.