Steps to reproduce:

1/ Start Orca.

2/ Start gtk-demo Tree View -> List Store demo.  
   The demo comes up with the "Bug Number" column header with focus.

3/ Arrow right twice to the "Description" column header.

4/ Arrow down twice.

5/ Arrow left once.
   Focus is on the "Critical" text in the Severity column.

6/ Press Shift-Tab. Focus is given to the "Severity" column header.

7/ Arrow down once.

Focus is back on the "Critical" text but Orca speaks:

SPEECH OUTPUT: ''
SPEECH OUTPUT: 'Fixed? column header'
SPEECH OUTPUT: 'check box not checked '

But it's the same problem. No "object:selection-changed" event,
so we are unable to setup the index of the child that can be used in
onFocus().

So what you've done certainly helps very much, but it looks like
there is another code path through gailtreeview.c that's going to
need to send this event.