Issue 4718/1: In-chord tie in cue causes segfault

Issue 4718/1: In-chord tie in cue causes segfault

This extends transposition functionality from Music expressions to
Prob in general since Stream_event also needs transposition in the
context of quoted music.  While at top level, Stream_event is
processed using the container-agnostic transpose_mutable, this does
not extend to the actual contents, and at least a Stream_event's
articulation list does not consist of Music expressions but again of
Stream_event elements.