Documentation about how to handle certificate errors was poor,
so this patch try to improve that.

[ChangeLog][QtWebEngineQML][QQuickWebEngineCertificateError] New API for handling certificate errors in the QML API.

Change-Id: I9234f5c8296651e7634ee41827fc6dcce503398b
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>