Move SRPAuth to the backend.

* 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]
Keep track of backend's connected signals.
Refactor EIPBootstrapper to the backend
Use helper to get the ProviderConfig.
Do not redefine signaler signal key.
Fix: use signal's keys instead of qt signals.
Move openvpn_verb to config.flags
Remove unused 'hack' helpers.
Move EIP to the backend.

- Add backend eip management
- Connect gui with new eip signals
- remove old unused code
- remove Qt dependency from backend services.
- use Signaler to emit status/state changes from openvpn
Add changes file for #5349.
Move gateway selection helpers to backend.
Handle trying to kill a non running process.
Merge branch 'refactor/move-eip-to-backend' into refactor/move-logic-to-backend