Add "checkable" state to accessible menu item

The information about whether a menu item may be checked is necessary
to allow the platform code (in particular, Windows UI Automation layer)
to make this information available to screen readers.

Task-number: QTBUG-80551
Change-Id: Ibfcc4f2da1ebc68e7dc5df2cd46bbfc0a177da12
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/accessible/qaccessiblemenu.cpp

@@ -299,6 +299,8 @@ QAccessible::State QAccessibleMenuItem::state() const
         s.disabled = true;
     if (m_action->isChecked())
         s.checked = true;
+    if (m_action->isCheckable())
+        s.checkable = true;
 
     return s;
 }