Implement a selection column, sortable columns, and row hovering in MailRecipe

Review at http://gwt-code-reviews.appspot.com/356801

Review by: jgw@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7936 8db76d5a-ed1c-0410-87a9-c151d255dfc7