a11y: Make AnnouncementPriority a Q_ENUM

It makes it possible to have it used from QML.

Change-Id: Ie4a1ef432f2a06e1d4bbe106bd41b5843e24d8bb
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

src/gui/accessible/qaccessible_base.h

@@ -372,6 +372,7 @@ public:
         Polite,
         Assertive
     };
+    Q_ENUM(AnnouncementPriority)
 
     typedef QAccessibleInterface*(*InterfaceFactory)(const QString &key, QObject*);
     typedef void(*UpdateHandler)(QAccessibleEvent *event);