Use a dedicated capf instead of rebinding 'C-M-i'

Use a dedicated capf instead of rebinding 'C-M-i'

Instead of rebinding 'C-M-i' to a command that inserts the suggested
completion, we make 'completion-at-point' do the right thing when the
preview is active by temporarily adding a prioritized capf.

Thanks to João Távora for suggesting this approach.

* completion-preview.el (completion-preview-active-mode-map): Remove.
(completion-preview--insert):
(completion-preview-insert-function): New functions.
(completion-preview-active-mode): Adjust.