TinyMCE:
- Use `data-wp-*` for processing and styling of the `more` and `nextpage` placeholders, keep the class names for back-compat. Makes them "immune" to "Clear formatting".
- Translate the titles of the placeholders.
Fixes #28772.
Built from https://develop.svn.wordpress.org/trunk@29317


git-svn-id: http://core.svn.wordpress.org/trunk@29098 1a063a9b-81f0-0310-95a4-ce76da25c4cd