Add an optional close button to the info bar

This adds gtk_info_bar_{get,set}_show_close_button.
Similar to the one for the search bar. This can replace Cancel
buttons.

https://bugzilla.gnome.org/show_bug.cgi?id=704274

gtk/gtkinfobar.h

@@ -124,6 +124,12 @@ void           gtk_info_bar_set_message_type       (GtkInfoBar     *info_bar,
 GDK_AVAILABLE_IN_ALL
 GtkMessageType gtk_info_bar_get_message_type       (GtkInfoBar     *info_bar);
 
+GDK_AVAILABLE_IN_3_10
+void           gtk_info_bar_set_show_close_button  (GtkInfoBar     *info_bar,
+                                                    gboolean        setting);
+GDK_AVAILABLE_IN_3_10
+gboolean       gtk_info_bar_get_show_close_button  (GtkInfoBar     *info_bar);
+
 G_END_DECLS
 
 #endif  /* __GTK_INFO_BAR_H__  */