a11y: Use desktop file name as app's accessible ID

As the QGuiApplication::desktopFileName doc [1] says:

> This is the file name, without the full path or the
> trailing ".desktop" extension of the desktop entry that
> represents this application according to the freedesktop
> desktop entry specification.

Since the desktop entry represents the application, also
use the desktop file name for the accessible ID of the
application object.

This e.g. makes Okular's application object report an
accessible ID of "org.kde.okular" instead of just "QApplication"
as the AT-SPI2 AccessibleId property on Linux.

[1] https://doc.qt.io/qt-6/qguiapplication.html#desktopFileName-prop

Change-Id: I554da7baaf817866db9044128f73a9ea634cddbf
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 13b4075093866a497e620bdd388afaf8aa0bc962)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/accessible/qaccessibleobject.cpp

@@ -182,6 +182,8 @@ QString QAccessibleApplication::text(QAccessible::Text t) const
         return QGuiApplication::applicationName();
     case QAccessible::Description:
         return QGuiApplication::applicationFilePath();
+    case QAccessible::Identifier:
+        return QGuiApplication::desktopFileName();
     default:
         break;
     }