Use AT-SPI screen reader status on Linux

[matthewflint]

Summary

Use org.a11y.Status.ScreenReaderEnabled for Linux screen reader mode detection instead of treating QAccessible::isActive() as equivalent to a running screen reader.

On affected Linux X11 setups, QAccessible::isActive() can become active even when no screen reader is running. That broader accessibility-active state is a poor match for Telegram's screen-reader-specific mode and causes false activation downstream.

What changed

• add a Linux-specific ScreenReaderState implementation based on AT-SPI status
• keep QT_LINUX_ACCESSIBILITY_ALWAYS_ON as an explicit override
• listen for runtime AT-SPI status changes through D-Bus
• avoid starting org.a11y.Bus just to probe the status
• stop using the generic QAccessible::isActive() fallback on Linux

Rationale

Applications sometimes need a narrower product signal than Qt's generic accessibility-active state. For Telegram Desktop, that signal feeds screen-reader-specific behavior and a user-facing warning bar. Using ScreenReaderEnabled is a better fit for that decision than using any accessibility backend activity as a proxy.

Validation

• syntax-checked the touched translation units with local Qt and GIO headers
• verified org.a11y.Status property reads against a live session bus
• verified PropertiesChanged delivery for runtime updates
• kept the change scoped to the Linux detector path only

[CLAassistant]

All committers have signed the CLA.

[ilya-fedin]

Qt's implementation needs a few lines fix and I would like to avoid having 213 lines of new C(!!!) code unless the problem is discussed with Qt folks. Also, you could notice other D-Bus code in the repo uses C++ codegen.

[matthewflint]

Thanks, got it. I'll take this to the Qt side first and reduce it to a minimal repro.