[FIX] web_kanban: properly removes nvd3 tooltips

with the recent update of nvd3 to the last version, we fixed a lot of
tooltip issues, but we introduced at least a new problem:  when a view
is detached from the dom with a visible tooltip, it no longer is cleaned
up.

In this commit, we simply remove all nvtooltips when we detach a kanban
view (necessary because dashboards inherits from kanban).  This is not
really an elegant way to solve the issue, but at least it works.  any
improvement is welcome.

addons/web/static/src/js/views/graph_view.js

@@ -19,6 +19,10 @@ var GraphView = View.extend({
     icon: 'fa-bar-chart',
     view_type: 'graph',
 
+    on_detach_callback: function () {
+        $('body > .nvtooltip').remove();
+    },
+
     init: function(parent, dataset, view_id, options) {
         this._super(parent, dataset, view_id, options);