Various changes:

- Bump racket-mode-version to 0.4.
- Move command dispatch sandbox.rkt to cmds.rkt.
- Add racket-visit-module.
- Rename racket-find-definition to racket-visit-definition.
- Rename racket-help to racket-doc.
- Reassign C-c C-d to racket-doc.
- Remove C-c C-h key binding (using C-h in a key binding is bad).
- Reorganize menu.