WTextEdit.js : iframe object doesn't have a contentDocument.document property, contentDocument is sufficient. (on Firefox, not tested elsewhere)

It was causing strange issues where :

- an error was logged in firefox JS engine, saying that the document property doesn't exist.

When creating another textarea object without reloading page, an alert dialog is displayed about this error (why only now?) and the page is no longer responding to events.

Seem to be working really well with this patch.