Respect UIA Notification processing flag (PR #9466)
In the Windows 10 may 2019 update, Microsoft decided to announce volume changes using UIA notifications. This means that these volume notifications are announced when the keyboard focus is in Explorer. However, due to the way NVDA handled UIA notifications before this pr, these volume change notifications were queued one after another, resulting into lots of notifications being announced, including duplicates.
Now UIA notifications have a notificationProcessing parameter. When this parameter is set to one of the most recent NotificationProcessing constants, speech is cancelled before announcing the new notification.
Tested with volume changes in Windows 10 1903.
