New user option 'completion-preview-idle-delay'

New user option 'completion-preview-idle-delay'

Support displaying the completion preview shortly after you
pause typing (on idle) rather than immediately.

* lisp/completion-preview.el (Commentary): Update.
(completion-preview--try-update):
(completion-preview--update-from-timer): New functions.
(completion-preview--timer): New buffer-local variable.
(completion-preview-idle-delay): New user option.
(completion-preview--show): Use it.
(completion-preview--post-command):
(completion-preview-mode): Disable idle timer if active.