scm/define-stencil-commands.scm: cleanup

scm/define-stencil-commands.scm: cleanup

- Create a new procedure for stencil "commands",
  the subset of stencil expressions that can be
  defined in output-*.scm.

- Add the missing `setrotation' and `resetrotation'
  expressions.

- Use ly:all-output-backend-commands for the rest
  of the stencil expressions; thus,
  `delay-stencil-evaluation' belongs here, and
  add the missing `rotate-stencil' expression.

- Document the differences between the two procedures.

- Register all of the expressions in "one go".