API REVIEW request for RT-17138 Allow promptText on TextArea controls

[Leif Samuelsson]

JIRA: http://javafx-jira.kenai.com/browse/RT-17138

This tweak will move the following API up from TextField to its superclass
TextInputControl, and will therefore also be available on TextArea.

     /**
      * The prompt text to display in the {@code TextInputControl}, or
      * <tt>null</tt> if no prompt text is displayed.
      */
     public final StringProperty promptTextProperty();
     public final String getPromptText();
     public final void setPromptText(String value);