[nemo-systemsettings] Add setCurrentUser method to UserModel. Contributes to JB#47825
Add updateCurrent to UserInfo. Checks if user is current and updates the status.
Add setCurretUser method to UserModel. Allows to logout and login to another user session using usermanager.
Add ErrorType enum to UserModel. Remove name argument from failure signals as it wasn't used anyway and that information can be obtained by knowing which row is affected.
Fix a mistake UserModel user removal handling. Uid mapping to rows was not correcty updated when removing users. Also improve QHash usage overall and always append to the end of userAddFinished.