Rename the problematic "unload" event in Thickbox to "tb_unload", making it a...

Rename the problematic "unload" event in Thickbox to "tb_unload", making it a true custom event, fixes #19189

git-svn-id: https://core.svn.wordpress.org/trunk@19456 1a063a9b-81f0-0310-95a4-ce76da25c4cd