Add containers to board view

issue10537
review367231002
(grafted from d9e05c64f3b3e5062a603782980aa6a58d4c6f8d)

--HG--
branch : 6.0

src/board.js

@@ -34,6 +34,7 @@
 
             this.context = context;
             this.actions = [];
+            this.containers = [];
             this.state_widgets = [];
             this.el = jQuery('<div/>', {
                 'class': 'board'