[KJobUiDelegate] Add AutoHandlingEnabled flag

Summary:
This allows to simplify code from

KDialogJobUiDelegate *delegate = new KDialogJobUiDelegate;
delegate->setAutoErrorHandlingEnabled(true);
job->setUiDelegate(delegate);

to

job->setUiDelegate(new KDialogJobUiDelegate(KJobUiDelegate::AutoHandlingEnabled));

(which include warnings, just in case)

Test Plan: Builds, requires a kjobwidgets change on top to be useful.

Reviewers: broulik, davidedmundson, ervin

Reviewed By: ervin

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28741