RemoteConfig: Refactoring

[rznccc]

Hello!

1. In case of Firebase CPP SDK >= 8.0.0, there was no control of futures returned from SDK code. So the correctness was not guaranteed.
2. In case of Firebase CPP SDK >= 8.0.0, no need to give up if Google Play Services is not available. They could be updating, for example, SDK defines a set of reasons and supports its API anyway. So let it decide.
3. Initial 500 ms delay has been removed from the initialization workflow: QTimer::singleShot(500 -> 0, ...).
4. Added new property fetching.
5. Added new signals googlePlayServicesError and futuresError.
6. Pooling future's status on timer is not an elegant and efficient solution, I used callbacks.
7. Support of Firebase CPP SDK < 8.0.0 has been kept.
8. Backward compatibility of QtFirebase API has been kept.

[larpon]

Very nice, thanks 👍🏻