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