Popup can't show candidates without selecting.

[kenoss]

This is based on conversation with @supermomonga.

In some cases, for example, using autocomplete.el, we want to show candidates without selecting one of them. By default, auto-complete.el force to select the first candidate and pressing Enter key expands the candidate.

What we really want is:

Enter and Tab: expand candidate if selected, otherwise self insert.
(Optional) Tab: always expand. If nothing selected, expand first one.

AFAIU, popup.el doesn't admit this. I think we'll easily implement the above (optional) feature to auto-complete.el if popup.el admits it. (We may need some additional APIs.) However the modification for it on popup.el is not straightforward, I guess.

What do you think of it?