stylecontext: add a raised style class

This is useful for raised elements, for example raised buttons on
primary toolbars.

https://bugzilla.gnome.org/show_bug.cgi?id=644248

gtk/gtkstylecontext.h

@@ -315,6 +315,14 @@ struct _GtkStyleContextClass
  */
 #define GTK_STYLE_CLASS_ACCELERATOR "accelerator"
 
+/**
+ * GTK_STYLE_CLASS_RAISED:
+ *
+ * A CSS class to match a raised control, such as a raised
+ * button on a toolbar.
+ */
+#define GTK_STYLE_CLASS_RAISED "raised"
+
 /**
  * GTK_STYLE_CLASS_GRIP:
  *