allow the TextAreaFormField to be used with valid/invalid HTML

The TextAreaFormField previously used saveXML to get a representation of the child nodes of an textarea.
This works pretty fine on simple text is causes issues in case you have broken HTML, as saveXML
will potentially also break encoding/change the structure.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT