enable all preview plugins
Summary:
try to preview everything that's found in the list.
by default not all image types are enabled, like
svg, which is a supported wallpaper type
Test Plan: svg files are correctly previewed in the thumbnail grid
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D3017