Themes for the CSS engine

Hello,

with basic support for the `border-image' property [1, 2] the CSS
engine is now almost feature complete for the 0.3 release. This means
theme designers can now start doing their thing, come up with new
looks, and expose bugs while going along. Documentation is sparse, but
if you know CSS things should be pretty intuitive. I'll try to hang
out in #gnome-art regularly, feel free to show up and discuss things.

[1] http://www.w3.org/TR/css3-background/#the-border-image
[2] http://www.gnome.org/~robsta/gtk-css-engine/screenshots/14-border-image.png

Best,
Rob