fdo#35105: notify role change through a new UNO a11y event

A new UNO accessibility event called ROLE_CHANGED has been created.
It should be triggered when an accessible object changes its role and
every accessibility toolkit should use its own methods to make that
change effective.

Code to support the event in ATK has been added.

Change-Id: I132e303bdb148967231334458c3cfa369c36ec8f
Reviewed-on: https://gerrit.libreoffice.org/7853
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>