[FIX] web: allow on_(attach|detach)_callback callbacks to be called

The `on_attach_callback` and `on_detach_callback` callbacks are
called when an a widget is attached / detached according to the
`is_in_DOM` variable. As the community's action manager cannot
be detached from the DOM, we can hardcode this value to true.

This allow to run these callbacks when needed.

addons/web/static/src/js/web_client.js

@@ -24,6 +24,10 @@ return AbstractWebClient.extend({
     },
     show_application: function() {
         var self = this;
+
+        // Allow to call `on_attach_callback` and `on_detach_callback` when needed
+        this.action_manager.is_in_DOM = true;
+
         this.toggle_bars(true);
         this.set_title();
         this.update_logo();