Editorial annotation improvements

• The FingerGlideSpanner may now connect StringNumber and StrokeFinger grobs.

  {
    \set strokeFingerOrientations = #'(down)
    b2 \glide -\rightHandFinger #1 \glide -2 \glide \5
    e'-\rightHandFinger #1 -2 \5
  }
  

  

• NoteName grobs are now horizontally centered by default.