[GLISS] Issue 2717: Implement \hide as a shorthand for \tweak #'stencil ##f

I want feedback on this one.  I lean slightly towards the course
outlined in comment 9
<URL:http://code.google.com/p/lilypond/issues/detail?id=2717#c9>.  But
this is a decision I should not be taking alone.

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David Kastrup