Overhaul Editor framework traversal logic to use a visitor pattern.
Add a base EditorDriver interface.
Add HasRequestContext interface for better RF+Editor integration.
Patch by: bobv
Review by: rjrjr, tbroyer

Review at http://gwt-code-reviews.appspot.com/1340802


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9666 8db76d5a-ed1c-0410-87a9-c151d255dfc7