Fix oauth focus

Summary:
Set the focus to the webview. Previously, if you entered an e-mail address
and pressed enter, it would have closed the webview, because the cancel button
is the default button. I think this is a QT bug, so I opened a QT bugreport.

BUG: 389263

Test Plan:

• In KMail, the focus works now
• In KOrganizer, the focus works now

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D10206