Drop support for SecureDesktopNVDAObject, replace with an extension point

[seanbudd]

Is your feature request related to a problem? Please describe.

SecureDesktopNVDAObject is used to notify the user and API consumers that NVDA has entered a secure desktop. This creates a valid NVDAObject, backed by the secure desktop. The running instance of NVDA enters sleep mode when SecureDesktopNVDAObject is focused, signifying a switch to a secure desktop. Then a new instance of NVDA in secure mode starts on the secure desktop. The SecureDesktopNVDAObject is unused, apart from being an API endpoint to notify consumers, as NVDA sleeps after it is created.

Handling SecureDesktopNVDAObject requires special cases to be constructed.

NVDA generally uses extension points for this API use case.

Describe the solution you'd like

Drop support for SecureDesktopNVDAObject, replace with an extension point

Describe alternatives you've considered

Continue support for SecureDesktopNVDAObject

Additional context

Considered due to the work in #14105, #14116