Change cursor type when showing completion preview

Change cursor type when showing completion preview

* lisp/completion-preview.el (completion-preview-cursor-type):
New user option.
(completion-preview--window): New internal variable.
(completion-preview--window-selection-change): Bind it.
(completion-preview--original-cursor-type): New local var.
(completion-preview-active-mode, completion-preview--show): Set
'cursor-type' window parameter according to
'completion-preview-cursor-type', or reset it.
(completion-preview--post-command): Reset cursor type in
'completion-preview--window' if last command switched windows.