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