To inform ATs about what is going on, implement AtkSelection,
and also emit ::active-descendant-changed. This seems enough
to make orca read 'filler' for each row (unless the row happens
to be just a label, in which case, it reads the label).
https://bugzilla.gnome.org/show_bug.cgi?id=692258
Focus in/out this window used to take a lot of CPU. See
https://bugzilla.gnome.org/show_bug.cgi?id=687911
The dummy row widget code is copied from empathy-roster-contact.c to
make the test more realistic.