- Use python `object` as a parent class.
- Replace QMutex with a `threading.Lock`

* Move methods from SRPAuth to backend: login, logout, change_password.

* Add backend section to hold temporary hack code, needed in the process
  of splitting frontend and backend.

* Replace pyside signals with Signaler signals.

* Move all the signaling and thread launching in SRPAuth inside of
  __impl.

* Move defer handling code (callbacks/errbacks) to the backend and left
  only signal handling in the GUI.

[Closes #5347]

Also:
- remove unused code.
- add status attributes for srpauth responses.
- split response handling
- remove unneeded hack to avoid segfault emitting None

Also refactor signals to be more granular and notify clearly if there
was an error or not, then in the GUI we show the corresponding message.

- Fix issues related to "Cancel login does not work".
- Move srpauth errback to mainwindow.
- Add signal for provider setup cancel.
- Add support to cancel the soledad defer.

[Closes #4869]
[Closes #4973]

This will skip:
* srp authentication with server
* remote soledad configuration
* keymanager sending key to server
* imap fetches.

Its main goal is to help us while debugging imap accounts, by cutting
almost all communication with server.

It will break havoc if you use it without having local keys configured.
So, basically, use with care.