Call javaScriptConfirm for unload dialogs

This unfortunately means we lose the ability to customize the title with the
default implementation, but it's still better than having a QMessageBox
hardcoded without being able to customize this behavior at all.

[ChangeLog][Qt WebEngineWidgets][QWebEnginePage] The javaScriptConfirm method is
now also called for unload dialogs (onbeforeunload handlers).

Task-number: QTBUG-58673
Change-Id: I82393da2593dc84e94d577ff28f5d6d8988df552
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>