TinyMCE: fix `aligncenter` for html5-captions, props obenland, fixes #28681.
Built from https://develop.svn.wordpress.org/trunk@29195


git-svn-id: http://core.svn.wordpress.org/trunk@28979 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Avoid a race condition when multiple windows are open so that orphaned terms cannot be created by accident.

Adds a unit test.

Props dlh.
Fixes #19205.

Built from https://develop.svn.wordpress.org/trunk@29196


git-svn-id: http://core.svn.wordpress.org/trunk@28980 1a063a9b-81f0-0310-95a4-ce76da25c4cd