Improvements #11272: WTextEdit / TinyMCE improvements
Supporting placeholders should be quite easy: just call the base implementation.
The textarea placeholder is supported from TinyMCE 5.2:
If the editor is initialized on a textarea element, the placeholder attribute can be used instead.
Not sure if we still need to support TinyMCE 4, one possibility is to only throw the error when TinyMCE 4 is used.