Introducing FieldTypeWriter, an enum that identifies field writers and
defines precedences useful for disambiguation when sorting builders
in the Widgets ctor.

Review at http://gwt-code-reviews.appspot.com/1428807


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10127 8db76d5a-ed1c-0410-87a9-c151d255dfc7