Notify screen readers when selection changes in listboxes

When you change selections in listboxes without dropdown fuctionality
an accessilble event needs to be sent, this does not happen today.
An example of such listbox is the Suggestions in the Spelling and
Grammar dialog.

Change-Id: I6100c88ae5398d8bf3f511aee7b35e88fbf151f5
Reviewed-on: https://gerrit.libreoffice.org/10087
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>