stylecontext: add an "image" style class

For images such as icons in GtkEntry widgets.

gtk/gtkstylecontext.h

@@ -400,6 +400,13 @@ struct _GtkStyleContextClass
  */
 #define GTK_STYLE_CLASS_SIDEBAR "sidebar"
 
+/**
+ * GTK_STYLE_CLASS_IMAGE:
+ *
+ * A widget class defining an image, such as the icon in an entry.
+ */
+#define GTK_STYLE_CLASS_IMAGE "image"
+
 /**
  * GTK_STYLE_CLASS_HIGHLIGHT:
  *