(completion--insert-strings): Split function; Full group title support

(completion--insert-strings): Split function; Full group title support

Split `completion--insert-strings` into a function per completions
format in order to increase readability and extensibility.  This
change eases the addition of more formats.  Add support for group
titles to the vertical and horizontal formatting functions.

* minibuffer.el (completion--insert): Add new function.
(completion--insert-vertical, completion--insert-horizontal,
completion--insert-one-column): Extract function from
`completion--insert-strings`.  Use new function `completion--insert`.
(completion--insert-strings): Use new insertion functions.