Removed the instance method get(Element element) method from the
Role.java and RoleImpl.java. Added a static method to Roles.java to get
the Role object from an Element.
Also removed Id.of(Widget) to avoid a circular dependency.

Fixes issue 7567

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11274 8db76d5a-ed1c-0410-87a9-c151d255dfc7