css: Implement font-stretch

css: Implement font-stretch

The font-stretch CSS property is defined in the Level 3 CSS Fonts
module, available at:

  http://dev.w3.org/csswg/css-fonts/#propdef-font-stretch

It allows defining a normal, condensed, or expanded face to the font
description. Pango already supports it, so this is literally just the
CSS parser machinery needed to bridge our CSS to the FontDescription
API.

https://bugzilla.gnome.org/show_bug.cgi?id=735593