Rename StThemeImage to StBorderImage

The current CSS3 border-image is close to a superset of what we were
doing for -hippo-background-image. Woot! rename StThemeImage to
StBorderImage and change parsing to look for:

 border-image: <url> <number>...

Rather than

 -st-background-image: <url> <length>...

percentanges for the border sizes are not currently supported, neither
are the keywords for handling of the middle part. We always do 'stretch'
for now.

https://bugzilla.gnome.org/show_bug.cgi?id=595990