Doc: Issue 3807: Maintaining \version in documentation files

  Add instructions on maintaining the \version string in doc files
Issue 3797: Music_wrapper_iterator does not follow original context setting of child

We do this by making Music_iterator::get_handle and
Music_iterator::set_context virtual and letting Music_wrapper_iterator
redirect them to the child iterator.

\score {
  <<
    { { a4 a b b } a a b b }
    { \absolute { a4 a b b } a a b b }
  >>
}

splits off a new Staff after \absolute { a4 a b b }.
Issue 3792: \time change under last \alternative does not persist