ntome authored Aug 06, 2020 and hydrargyrum committed Sep 09, 2020

PFMs need to keep track of accounts, which is done using various generic
methods. However, complicated cases may occur on some sites, where no generic
method works, without harming other modules.

A new capability method is introduced to allow module-specific diff behavior.
Essentially, a diff consists in:
- matching accounts, pairs of new/old objects
- new accounts, that were not seen before and are known to be new
- obsolete accounts, that are known to have disappeared
- unknown accounts, which could simply not be matched, but may be mapped by
  another method or manually by a user