Fix message hooking regression (#14789)
Follow up of #14759
Fixes #14784
Partially reverts 9b97de0
Summary of the issue:
At least one custom IME is reported to no longer work in editable controls within NVDA.
Description of user facing changes
Fix the regression
Description of development approach
We no longer use the background thread to handle cancellable messages, as it seems using the background thread causes IME messages to be discareded. This means a partial revert of 9b97de0. In fact, this pr removes the background thread but keeps the event based approach to cancel calls from NVDA's watchdog.
