Keep in sync with the current tree.
The changes are mostly caused by updates in the internal hierarchy of
composite widgets, and the fact that the order in which the widget tree
is traversed is not exactly stable.
This allows to override the role declared to the atk stack. For
instance,
<accessibility>
<role type="static"/>
</accessibility>
allows to tell the accessibility stack that a label is just a message in
a message box.
Fixes#109