can't delete placeholders

[porjo]

I'm using the placeholder plugin: https://ckeditor.com/cke4/addon/placeholder

When I delete text that contains a placeholder, the placeholder widget gets excluded from track changes i.e. the placeholder remains visible, and the source code shows it sitting outside the <del></del> tags.

Steps to reproduce:

1. start with editor that contains normal text and a placeholder
2. Delete all content in the editor
3. Click 'Source' button twice to toggle between source and back to rendered display (simulate reload of content)
4. Observe that placeholder deletion is not tracked. Viewing source shows it sitting outside <del> tags

I've created a reproduction of the issue here: https://pace7.com/ckeditor-issue/ **

** that's using latest track changes source from src/dist/lite and ckeditor v4.8.0 (with bundled placholder plugin)