widget: Add :focus-on-click property

There are currently three widget that implement such a property, and
there are other widgets for which the behavior can make sense. It
seems like a good time to add the property to GtkWidget itself so
subclasses can choose to respect it without adding their own property.

https://bugzilla.gnome.org/show_bug.cgi?id=757269

gtk/gtkwidgetprivate.h

@@ -56,6 +56,7 @@ struct _GtkWidgetPrivate
   guint sensitive             : 1;
   guint can_focus             : 1;
   guint has_focus             : 1;
+  guint focus_on_click        : 1;
   guint can_default           : 1;
   guint has_default           : 1;
   guint receives_default      : 1;