adding hideKeySignature to lilypond commands

[Graham Percival]

To make lilypond bagpipe stuff easier, I propose that we include these
commands in ly/property-init.ly
Any objections?  Should they go in a different file instead?


hideKeySignature = {
  \override Score.KeySignature #'print-function = ##f
  \set Score.extraNatural = ##f
  #(set-accidental-style 'forget)
}
showKeySignature = {
  \override Score.KeySignature #'print-function = 
#'Key_signature_interface::print
  \set Score.extraNatural = ##f
  #(set-accidental-style 'forget)
}