[FIX] web: properly destroy nvtooltips when switching views

If the graph view was left while a nvtooltip was shown (thanks to
keyboard shortcuts or during loading time), the nvtooltip remained
and was only destroyed when leaving the action.

Now they are automatically removed with the view as they are now
placed inside of them instead of inside the body element.

addons/web/static/src/css/base.css

@@ -3056,3 +3056,7 @@ body {
   white-space: nowrap;
   padding-left: 10px;
 }
+
+.nvtooltip thead {
+  background-color: transparent;
+}

addons/web/static/src/css/base.sass

@@ -2605,3 +2605,6 @@ body
         width: 100px
         white-space: nowrap
         padding-left: 10px
+
+.nvtooltip thead
+    background-color: transparent

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

@@ -97,7 +97,8 @@ return Widget.extend({
                     "there is no active filter in the search bar."),
             }));
         } else {
-            this['display_' + this.mode]();
+            var chart = this['display_' + this.mode]();
+            chart.tooltip.chartContainer(this.$el[0]);
         }
     },
     display_bar: function () {
@@ -181,6 +182,8 @@ return Widget.extend({
         chart(svg);
         this.to_remove = chart.update;
         nv.utils.onWindowResize(chart.update);
+
+        return chart;
     },
     display_pie: function () {
         var data = [],
@@ -231,6 +234,8 @@ return Widget.extend({
         chart(svg);
         this.to_remove = chart.update;
         nv.utils.onWindowResize(chart.update);
+
+        return chart;
     },
     display_line: function () {
         if (this.data.length < 2) {
@@ -308,7 +313,9 @@ return Widget.extend({
 
         chart(svg);
         this.to_remove = chart.update;
-        nv.utils.onWindowResize(chart.update);  
+        nv.utils.onWindowResize(chart.update);
+
+        return chart;
     },
     destroy: function () {
         nv.utils.offWindowResize(this.to_remove);