Change popups: resize horizontally and center vertically

[dwcoates]

This change makes board image centered vertically in popup. It also resizes the popup window horizontally to fix to image.

I could not get the image to center horizontally.

You might not notice a difference without a emacs -Q

[dwcoates]

It seems to create window too early, so it hiccups on the first call. I should fix that

[dwcoates]

A work in progress. Later, I'll try to make it popup below/right intelligently.

[rolandwalker]

The variable win in (let ( … (win (get-buffer-window buf)) … was perhaps badly named. It's expected to routinely be nil — when the window doesn't exist yet. That way setting window properties only such as (set-window-dedicated-p win t) happens only on the first display.