[FIX] web: hiding of o2m view manager header

One2many fields bring in the entire viewmanager widget.
When there is only one view though (e.g. only kanban view for contact
tab of partners), we want to hide the viewmanager header because we
don't need it (it's where the view switcher would go).

Odoo had the widget logic to do that, but it was wrongly implemented,
resulting in a always present oe_view_manager_header div with all
its elements (although empty).
The wrong javascript operation should have triggered a client error
when loading the view, but for pure chance it didn't because the
t-jquery selector was wrong too so the javascript was never evaluated.

addons/web/static/src/xml/base.xml

@@ -1539,8 +1539,8 @@
 </t>
 
 <t t-name="One2Many.viewmanager" t-extend="ViewManager">
-    <t t-jquery=".oe-view-manager-header">
-        this.attr('t-if', 'views.length != 1');
+    <t t-jquery=".oe_view_manager_header">
+        this.attr('t-if', 'widget.views_src.length != 1');
     </t>
 </t>
 <t t-name="One2Many.formview" t-extend="FormView">