Add support for a new -st-shadow property, which is based loosely
on the CSS3 box-shadow property:
http://www.css3.info/preview/box-shadow/
It defers from the specification as follows:
* no multiple shadows
* the optional color argument may be placed anywhere
* the shape is not determined by the widget's bounding box,
but by the background-image property
https://bugzilla.gnome.org/show_bug.cgi?id=603691
