I'm attaching yet another updated patch (v3), the difference compared to
the former patch is that now the alignment of completion candidate
annotations is based on `string-pixel-width` and `display` text
properties instead of padding with spaces.

This deals better with more exotic strings that may appear in the link's
title (again gnu.org is a good example here, as there are alternate link
titles in different languages).  Still I wonder if there's a more
principled way to align such annotations, any suggestions?