Fix opening the New Mail Notifier agent configuration

Summary:
This got accidentally broken by removing the showConfigurationDialog() method
from the resource's DBus interface. Instead of relying on DBus, use the new
in-process configuration dialog tooling.

Test Plan:
Clicking 'Other Actions' button in KMail's receiving account configuration
tab works again

Reviewers: mlaurent, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20962