fdo#39944: improve mapping between ATK and UNO roles

* UNO role GROUP_BOX was not mapped to any ATK role, now it's mapped
  to ATK_ROLE_GROUPING.

* UNO role COMMENT was mapped to ATK_ROLE_SCROLL_PANE, we replaced it
  with ATK_ROLE_COMMENT.

We use registerRole method to map these instead of doing a direct
association to prevent a failure in case the installed version of ATK
doesn't contain these roles.

Change-Id: I7e266a7e92d59b282c51934df05b53414588cc7e