TextField and TextArea should have placeholderText set by default, not text

When dragging and dropping TextField or TextArea onto the form editor, they have the text property set. This means that any changes to placeholderText will not be seen in the form editor or live preview, as text is always displayed if it exists. The Design Studio tutorials that Leena is working on has instructions to change placeholderText, which will have no visible effect currently.

It would be more user friendly to only set a default placeholderText.