Allow style plugins to properly access $form_state in submit and validate callbacks.

Style plugins can't access the $form_state array in the settings form's submit and validate callbacks. Plugin sometimes needs to do this. One example would be if you use input formats on some element in your settings form. The system doesn't recognize the selected filter as a setting and doesn't therefore put the value in the settings array. The only way to save these values are to access the $form_state array directly.

This will also make the submit and validate callbacks more similar to those used by other plugin types (contexts etc.).