[alarms] Fix Qt5 version of AlarmHandlerInterface::activeDialogs()

Exposing QObjectList to QML in Qt5 is not supported, use QList<Qobject *> instead (as suggested by rburchell, thanks!).

In addition, use QObject as AlarmHandlerInterface base class, no need for QDeclarativeItem/QQuickItem as the there's no visual stuff in AlarmHandlerInterface.