Tidy up filters

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@800411 b8457f37-d9ea-0310-8a92-e5e31aec5664
Check for duplicate slides being added to slideshow

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@800413 b8457f37-d9ea-0310-8a92-e5e31aec5664