at-spi: Filtering out unwanted window events

Hi,

I reported https://bugzilla.gnome.org/show_bug.cgi?id=776854 earlier,
and while I'd be happy to try and help providing a patch, I'm not sure
where to start looking.

The patch for the very old
https://bugzilla.gnome.org/show_bug.cgi?id=108664 seemed relevant, but
apparently the code is so totally different now it doesn't seem to
provide any useful insight.

If anyone has insight on how to fix this, it'd be much appreciated :)

Thanks!
Colomban