Use 16 as default size for checkbutton indicator

So that 16x16 icons can be used from themes such as Adwaita in their
natural size.

gtk/gtkcellrenderertoggle.c

@@ -70,7 +70,7 @@ enum {
   PROP_INDICATOR_SIZE
 };
 
-#define TOGGLE_WIDTH 13
+#define TOGGLE_WIDTH 16
 
 static guint toggle_cell_signals[LAST_SIGNAL] = { 0 };
 

gtk/gtkcheckbutton.c

@@ -35,7 +35,7 @@
 #include "gtkintl.h"
 
 
-#define INDICATOR_SIZE     13
+#define INDICATOR_SIZE     16
 #define INDICATOR_SPACING  2
 
 

gtk/gtkcheckmenuitem.c

@@ -34,7 +34,7 @@
 #include "gtkprivate.h"
 #include "gtkintl.h"
 
-
+#define INDICATOR_SIZE 16
 
 struct _GtkCheckMenuItemPrivate
 {
@@ -130,7 +130,7 @@ gtk_check_menu_item_class_init (GtkCheckMenuItemClass *klass)
                                                              P_("Size of check or radio indicator"),
                                                              0,
                                                              G_MAXINT,
-                                                             13,
+                                                             INDICATOR_SIZE,
                                                              GTK_PARAM_READABLE));
 
   widget_class->draw = gtk_check_menu_item_draw;