Add 'image_add_caption_text' filter for changing the pre-parse caption text before output to the editor.

props collinsinternet, DrewAPicture.
fixes #29607.
Built from https://develop.svn.wordpress.org/trunk@29753


git-svn-id: http://core.svn.wordpress.org/trunk@29525 1a063a9b-81f0-0310-95a4-ce76da25c4cd