Treat popup value property when completing candidate

To manually test, execute this:

```cl
(defun actest-value-candidates ()
  ;; (name . value) -- see `ac-candidates-1'
  '(("foofoo" . "foo")
    ("foobar" . "bar")))

(ac-define-source actest-keywords
  '((candidates . (actest-value-candidates))))

(progn
  (pop-to-buffer (get-buffer-create "*actest*"))
  (auto-complete-mode +1)
  (erase-buffer)
  (insert "fo")
  (run-with-timer 0.1 nil #'auto-complete '(ac-source-actest-keywords)))
```

fixes #148