Bug#870265: Try to reduce "unnecessary" children-changed event emission

[Alex ARNAUD]

Package: firefox
Version: 54.0-2
Tags: a11y
Owner: bugs at hypra.fr
User: bugs at hypra.fr
Usertags: hypra
Forwarded: https://bugzilla.mozilla.org/show_bug.cgi?id=1251992

CONTEXT:
On rare sites, the Orca screen reader becomes completely unusable due to 
the huge amount of events sent by Firefox. I don't reproduce the issue 
on Windows with NVDA so I suspect the issue has been solved on Windows.

DESCRIPTION FROM UPSTREAM:
"Steps to reproduce:
1. Load the attached pyatspi accessible-event listener in a terminal
2. Load/reload matrix.org

Desired results: Not 450+ object:children-changed:add:system events for 
images adding child images. :)

(I don't know if that's possible or not. But the act of examining these 
events in Orca just to rule them out as event spam still takes more time 
and a performance hit that is far from ideal. So if the spam could be 
detected and fewer events emitted, I'd be grateful.)"