Add filter for checkbox settings

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@958678 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add some filters to allow thumbnails to be cropped as well as the main images

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@958679 b8457f37-d9ea-0310-8a92-e5e31aec5664