Fix JavaScript signal connect on alias without other handlers

Fix JavaScript signal connect on alias without other handlers

The changed handlers for aliases are connected lazily in the engine.
QQmlPropertyPrivate::flushSignal is responsible for that and called in other
places, for example when installing a onSomeAliasPropertyChanged handler.
However we were missing a call to flushSignal when doing
onSomeAliasPropertyChanged.connect(...), i.e. using the JavaScript connect API.

Task-number: QTBUG-30493
Change-Id: Ia3f008626fd7af3f2cfbdd30d13fb83158bed4d5
Reviewed-by: Albert Astals Cid <albert.astals@canonical.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>