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

Exposing QObjectList to QML in Qt5 is not supported, use QQmlListProperty instead.

In addition, use QObject as AlarmHandlerInterface base class, no need for QDeclarativeItem/QQuickItem as the there's no visual stuff in AlarmHandlerInterface.

The hack for defining AlarmDialogList in src/alarmhandlerinterface.h is needed as Q_PROPERTY seems to get expanded even if ifdeffed out causing the moc file moc_alarmhandlerinterface.cpp in Qt4 to include references to QQmlListProperty.