Provide a core method to find out what templates are addable under a given parent #567
Description
It would be helpful if the core provided a method to find out what templates may be used for a page added under a given parent.
At first glance it seems like ProcessPageAdd::getAllowedTemplates() and ProcessPageAdd::isAllowedTemplate() might be useful for this purpose but they actually give incorrect results outside the context of ProcessPageAdd::execute(), as illustrated below.
The "news" template only allows children using the "news_item" template:
Incorrect results using ProcessPageAdd methods:
