icomplete-vertical simplest approach

icomplete-vertical simplest approach

* lisp/icomplete.el (icomplete--vertical-prospects) : New functions to
assert format and correct number of candidates in vertical format.
(icomplete--horizontal-prospects) : New functions to assert format and
correct number of candidates in horizontal format.
(icomplete-minibuffer-setup-hook) : Conditionally initialize the
variables for vertical format.
(icomplete-completions) : Simplify to separate some calculations
unneeded in vertical format. Moved part of its code to
icomplete--horizontal-prospects.