[orca-list] On Thunderbird, Orca doesn't announce the collapse/expended state of folders

[Alex ARNAUD <alexarnaud hypra fr>]

Hello all,

Environment:
- Debian Stretch with latest a11y stack
- Orca master
- Thunderbird 52.8.0

Steps to reproduce:
1) Launch Thunderbird
2) On the folder pane, move between folders with up/down arrow
3) There is normally expended/collapsed folder you can move on with 
up/down arrow

Result: Orca doesn't announce the collapsed/expended state of 
Thunderbird folders

Expected: Orca should announce the collapsed state as it does on folders 
of the list view of Caja.

Best regards.
--
Alex ARNAUD
Visual-Impairment Project Manager
Hypra - "Humanizing technology"