Improve prompt buffer UI on corner suggestions cases

[aadcg]

Ensure that the cases below are handled.

1

Notify user that suggestions are being computed:

(prompt :prompt "Test"
        :sources (make-instance 'prompter:source
                                :name "Test"
                                :constructor (lambda (x) (sleep 4) "one")))

---

2

Notify user that suggestions are being computed and finally that none are available:

(prompt :prompt "Test"
        :sources (make-instance 'prompter:source
                                :name "Test"
                                :constructor (lambda (x) (sleep 4) nil)))

---

3

Notify users that no suggestions are available:

(prompt :prompt "Test"
        :sources (make-instance 'prompter:source
                                :name "Test"
                                :constructor '()))

---

4

Notify user that no suggestions are available when inputting non-matching input such as "four".

(prompt :prompt "Test"
        :sources (make-instance 'prompter:source
                                :name "Test"
                                :constructor '("one" "two")))