fix: Watch the IsEnabled property to lazily activate on Unix

[DataTriny]

Closes #713

Following this commit in at-spi2-core we need to stop relying on the now removed ScreenReaderEnabled bus property and switched to monitor IsEnabled instead, which should be more reliable.

[DataTriny]

I'm not currently able to validate this on a Freedesktop environment.

@RastislavKish it would be great if you could apply lazy-activation-debug.patch on top of this branch then try the simple winit example. Let me know if the tree gets initialized immediately when Orca was previously running, if it can be first initialized later if Orca is not initially running and if it can be deactivated and reactivated during the app lifecycle.

Thank you for your help.

[RastislavKish]

@DataTriny looks good! I have installed at-spi-2-core from source and things seem to be working well, I can see both activation and deactivation reports as I turn on / off the screenreader. Everything also works as expected regardless when the screenreader is started i.e. before or after launching the app. Great work on both your and Mike's side!