Only present notifications when state changed to showing
requested to merge michaelweghorn/orca:michaelweghorn/only_present_notifications_when_showing into master
When the "showing" state of a notification changed, only present/announce it if the state changed so that it is showing now, not when it is no more showing.
Related issue: #12 (closed)
(s. related WIP LibreOffice change at https://gerrit.libreoffice.org/c/core/+/139709/3 which would otherwise result in the notification being announced twice, since an event is sent when the label that has the notification role gets hidden and when it gets shown)