XWiki Notifications alert too often

Here is an example described by a user in the notifications tab of profile edition:

```
When users change the frequency on the combo box list, there is then an alert repeating it is saved (several time) but not say what is saved:
“alert
alert Your settings are being saved...
Saved!
alert Saved!”
```

My guess is that there's an issue with how the XNotifications (https://github.com/xwiki/xwiki-platform/blob/9fb3d668265fdc7e9c8cfc42eea1a7e230e85e46/xwiki-platform-core/xwiki-platform-web/xwiki-platform-web-war/src/main/webapp/resources/uicomponents/widgets/notification.js#L87) are read out by screen reader (here NVDA). We want to change it so that it's read out more clearly.