No more massive copy paste between our request
objects. Introduces the abstract classes for RequestFactory
and its request object.

Also some clean up to make it more clear what classes will
come from GWT code generators and what will come from
pre-compilation tools. Part of this moves HandleManager to a
more appropriate package, since it won't be generated.

Review at http://gwt-code-reviews.appspot.com/250801

Review by: amitmanjhi@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7759 8db76d5a-ed1c-0410-87a9-c151d255dfc7