Reworked featured image importer

git-svn-id: https://plugins.svn.wordpress.org/easy-digital-downloads-csv-manager/trunk@953170 b8457f37-d9ea-0310-8a92-e5e31aec5664
Two of these files don't belong...

git-svn-id: https://plugins.svn.wordpress.org/easy-digital-downloads-csv-manager/trunk@953171 b8457f37-d9ea-0310-8a92-e5e31aec5664