Issue 2717: Implement \single, \omit and \hide (issue 6495135)

[k-ohara5a5a]

Works well for me.

You mis-typed something in the description because

> so you can use things like \single\voiceOne c4 .

is not true.  I can apply \single to simple sets of overrides, though,
like \single\voiceOneStyle


http://codereview.appspot.com/6495135/diff/2001/ly/music-functions-init.ly
File ly/music-functions-init.ly (right):

http://codereview.appspot.com/6495135/diff/2001/ly/music-functions-init.ly#newcode677
ly/music-functions-init.ly:677: #{ \override $grob #'stencil = ##f #})
I also tried this with #'stencil = #point-stencil
which avoids some programming-error messages when we \omit NoteHead.

It is more useful as you have it, with #'stencil = ##f
because the point stencil affects spacing of other things that are
padded away from the point-stencil.  I can no longer find any cases that
crash.

http://codereview.appspot.com/6495135/