Make "Default Applications" in mimeapps.list the preferred applications

Summary:
Use mimeapps.list "Default Applications" to prepend the service to the offers for this mimetype.
So that KMimeTypeTrader::preferredService will respect the setting.

Use KMimeTypeTrader::preferredService to replace usage of KEmailSettings / emaildefaults.

Specs:
https://specifications.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html
https://specifications.freedesktop.org/mime-apps-spec/mime-apps-spec-latest.html

CCBUG: 403499
FIXED-IN: 5.67

Reviewers: dfaure, dvratil, ervin, Frameworks

Reviewed By: dfaure, ervin

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26690