Divided the capabilities view into groups

As requested in #1934 a new pull request against the 3.0 unstable branch

I rewrote the way capabilities get divided into groups see commit: bad7188f371b873142cabd030d5b081eb5e34573
Pretty happy with the end result but it would be way better if WordPress adds groups to capabilities by default :sweat_smile: 

Due to the rewrite Gravityforms is not yet supported