Remove FieldWriter.setAttachable() and find out automatically whether the callbacks should be created.

Review at http://gwt-code-reviews.appspot.com/1421807

Review by: rjrjr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10090 8db76d5a-ed1c-0410-87a9-c151d255dfc7