Add Qt WebEngine Widgets API for allowing certificate errors

This adds API for overriding some certificate errors. Once overridden
any identical error for the same hostname and certificate will use
the same override.

Similar API for QtWebEngine QML should be added in a later patch.

Change-Id: I144147b86d9b592e3f87346a1e48890acee0c670
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>