Implement -st-shadow for StWidget

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