Scoped lookup of music properties

Add a protected internal_get_property () method allowing a music iterator to look up a property of its own music or enclosing music.

Get repeat-count by scoped lookup. This might not be enough on its own to justify the complexity of scoped music-property lookups, but it's a nice simplification.

The main use case is a planned \fine command that will work differently inside a folded \repeat than outside. The existing \unfolded feature is not sufficient for this because \unfolded takes effect when the immediately enclosing repeat is unfolded, but \fine needs to remain dormant until all enclosing repeats are unfolded.