[nemo-systemsettings] Add transitioning property to UserModel. JB#49822
This property tells if user is being created or deleted. This depends on whether user is placeholder (i.e. doesn't exist but can be created) or not (exists and can be removed).
Additionally use uid in callbacks instead of row numbers. Rows can changed, uids stay and we have a nice mapping that can tell which row to use.
Expose number of existing users and maximum number of users and also add new error enum for trying to go over maximum number of users.
Remove a useless comment line.