bug#19355: 25.0.50; limit the popup window height

[martin rudalics]

> The popup window from, for example, minibuffer-completion-help, can be
> so tall that the main editing window is squeezed to a few lines

This number of lines is specified by `window-min-height'.

> which
> has the unfortunate side-effect of moving point, pushing some buffer
> text out of the window. This is quite an annoyance. To see this:
>
> 1. Find an elisp file, with the form under edit centred around the
> window.
>
> 2. Try complete for `(s' and assume the window split will be done
> vertically.

We can add the MAX/MIN-HEIGHT/WIDTH arguments of `fit-window-to-buffer'
to the alist entries recognized by `display-buffer'.  Then
`minibuffer-completion-help' and friends could set them in a way we'd
have to choose yet.  Or we make these globally customizable.

I'm currently rewriting the code for *Completions* windows to make sure
they don't get obscured by an expanding minibuffer window.  If you have
a good proposal I can make the necessary changes right away.

martin