Move \RemoveEmptyStaves to new file for context modifications.

Move \RemoveEmptyStaves to new file for context modifications.

This allows \RemoveEmptyStaves to be invoked outside \layout blocks, i.e.,
directly following a context instantiation:

\new Staff \RemoveEmptyStaves { ... }

or

\new Staff \with { \RemoveEmptyStaves } { ... }

* input/regression/remove-empty-context-mod.ly

  new regtest

* ly/context-mods-init.ly:

  new file for context modification identifiers

* ly/declarations-init.ly:

  include context-mods-init.ly; placed before engraver-init.ly to ensure
  \RemoveEmptyStaves is accessible for deprecated \RemoveEmptyStaffContext
  and analogues

* ly/engraver-init.ly:

  remove \RemoveEmptyStaves declaration