Read only pseudo class is misspelled in GenericStyledArea

[Jugen]

The read only pseudo class parameter in GenericStyledArea is currently read-only but to be consistent with CSS convention should be readonly (i.e. without the hyphen).

The misspelling causes the RichTextFX area to not respond to the CSS :readonly pseudo‑class.

[JordanMartinez]

That's my fault. It originally did not have the hyphen, but I added it because I thought it was breaking convention. Mind submitting a PR for that?