Use KIO::PreviewJob::defaultPlugins()

Summary:
Use KIO::PreviewJob::defaultPlugins() instead of hardcoding plugin list.

Depends on D8347

Test Plan: Compiles fine in KDE Neon.

Reviewers: #frameworks, broulik, elvisangelaccio, markg

Reviewed By: markg

Differential Revision: https://phabricator.kde.org/D8352