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: Frameworks, elvisangelaccio, markg
Differential Revision: https://phabricator.kde.org/D8352