Fix bug where NVDA would not speak until restarted after add-on installation. (#15448)
Fixes #14525
Summary of the issue:
NVDA blocks exit on modal dialogs which require an answer.
If a message box is created after NVDA has triggered a shutdown, NVDA blocks exit on the message box, even after synthesizers have been terminated.
Description of user facing changes
Fix bug where NVDA would not speak until restarted after add-on installation.
Similar bugs with the same root cause may also be fixed.
Description of development approach
Open, pending modal dialogs should block the exit of NVDA.
To prevent a deadlock NVDA should prevent opening new modal dialogs if the core shutdown has been triggered.