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

src/st/st-theme-node.h

@@ -3,7 +3,7 @@
 #define __ST_THEME_NODE_H__
 
 #include <clutter/clutter.h>
-#include "st-theme-image.h"
+#include "st-border-image.h"
 
 G_BEGIN_DECLS
 
@@ -126,10 +126,7 @@ StTextDecoration st_theme_node_get_text_decoration (StThemeNode *node);
  */
 const PangoFontDescription *st_theme_node_get_font (StThemeNode *node);
 
-/* This is the getter for -st-background-image, which is different from
- * background-image in having provisions for unscaled borders.
- */
-StThemeImage *st_theme_node_get_background_theme_image (StThemeNode *node);
+StBorderImage *st_theme_node_get_border_image (StThemeNode *node);
 
 /* Helpers for get_preferred_width()/get_preferred_height() ClutterActor vfuncs */
 void st_theme_node_adjust_for_height       (StThemeNode  *node,