Updated core TinyMCE table plugin from 4.0.20 to 4.0.21 in sync with WordPress - mostly bug and accessibility fixes. Refactored for compatibility with plugins like Advanced Custom Fields that do not use the_editor hook

git-svn-id: https://plugins.svn.wordpress.org/mce-table-buttons/trunk@924343 b8457f37-d9ea-0310-8a92-e5e31aec5664