Use GTask based AsyncInitiables classes

Use GTask thread to initialize objects asynchronously to prevent that blocking operations might affect the UI (as per issue #239 (closed)).

Ideally I imagine all the other GSimpleAsyncResult should be moved to GTask but this wasn't the point of this branch.

I moved all the objects to use GTask as well natively, although I think this part can even be optimized more by ensuring that the whole creation happens in a different thread and finally we finish on main thread. But for now, I'd prefer to keep things as this.