Ask confirmation to unload the page

If there are any tabs open, we should ask confirmation to prevent user to
loose unsaved data.
We should also save the tree state of the menu like it is done on logout.

issue6756
review44301002

src/sao.js

@@ -80,6 +80,17 @@ var Sao = {};
         })()
     });
 
+    window.onbeforeunload = function(e) {
+        if (Sao.main_menu_screen) {
+            Sao.main_menu_screen.save_tree_state(true);
+        }
+        if (Sao.Tab.tabs.length) {
+            var dialog = Sao.i18n.gettext("Are your sure to leave?");
+            e.returnValue = dialog;
+            return dialog;
+        }
+    };
+
     Sao.class_ = function(Parent, props) {
         var ClassConstructor = function() {
             if (!(this instanceof ClassConstructor))