completion-preview.el: Provide an API.

completion-preview.el: Provide an API.

Add new functions for summoning and dismissing the
completion preview programmatically.  This lets Lisp code
leverage the completion preview UI for different use cases.

* lisp/completion-preview.el (completion-preview-show): New
function.
(completion-preview-hide): Rename to
'completion-preview--hide', use old name for new public API
function.
(completion-preview--hide): New name, see above.
(completion-preview-active-mode)
(completion-preview--update): Use new functions.