Allow to chose a notification template for mentions in specific xobjects

Right now mentioning someone in an xobject lead to a standard notifications, which will only inform about a mention in an "AWM field" with the specific location of the document containing this xobject.
Now when building an extension, we might want to be able to have custom notification templates for mentions in a specific xobject, that's particularly true when the location of the xobject storage is not the location of the sheet of the page.