; Avoid 'completion-at-point' in 'completion-preview-insert'

; Avoid 'completion-at-point' in 'completion-preview-insert'

Insert the completion suggestion directly in
'completion-preview-insert' instead of using 'completion-at-point' to
do that.  This fixes an issue where 'completion-preview-insert' would
not work correctly when the user uses 'add-hook' with a DEPTH argument
below a certain value to add functions to
'completion-at-point-functions', and obviates the need to manipulate
'completion-at-point-functions' when showing the preview all together.

* lisp/completion-preview.el (completion-preview--make-overlay)
(completion-preview-prev-candidate)
(completion-preview-next-candidate)
(completion-preview-mode): Improve docstring.
(completion-preview--exit-function)
(completion-preview--insert)
(completion-preview-insert-on-completion): Remove, no longer used.
(completion-preview--get): Turn into a 'defsubst'.
(completion-preview-active-mode)
(completion-preview--capf-wrapper): Simplify.
(completion-preview--try-table)
(completion-preview--update): Keep the completion "base" as a property
of the preview overlay, for use in completion exit functions.
(completion-preview-insert): Insert completion and call exit function
directly instead of manipulating 'completion-at-point' to do so.
(Bug#67275)