Store CSS classes and pseudo-classes pre-split in the StThemeNode

Tokenizing every time we want to evaluate CSS seems a poor plan.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=687465
Reviewed-by: Jasper St. Pierre <jstpierre@mecheye.net>

src/st/st-theme-node-private.h

@@ -78,7 +78,9 @@ struct _StThemeNode {
   GType element_type;
   char *element_id;
   char *element_class;
+  GStrv element_classes;
   char *pseudo_class;
+  GStrv pseudo_classes;
   char *inline_style;
 
   CRDeclaration **properties;