Issue 2859: Provide \hide and \omit functions for transparent and void glyphs

Issue 2859: Provide \hide and \omit functions for transparent and void glyphs

Both functions take either a grob name to override, or a music
expression to tweak (that is, the type of the argument decides whether
this results in an override or a tweak).

\hide sets #'transparent for the affected grob to ##t,
\omit sets #'stencil for the affected grob to ##f.

Example uses are
\new Voice \with { \omit StringNumber } { c'\4 }
{ <c' \hide g'~> g' }