support sorting only on enabled columns, fix empty table bug

git-svn-id: https://plugins.svn.wordpress.org/dbview/trunk@922197 b8457f37-d9ea-0310-8a92-e5e31aec5664
handle checkboxes needed to disable/enable column sorting

git-svn-id: https://plugins.svn.wordpress.org/dbview/trunk@922199 b8457f37-d9ea-0310-8a92-e5e31aec5664