improving cheatsheet appearance

[Mark Polesky]

In cheatsheet.itely

I propose changing line 70 (in "time signature") from
\override Staff.Clef #'transparent = ##t
to
\override Staff.Clef #'stencil = #empty-stencil

Also, in "key signature" (lines 95-102), 
the auto-generated \paper block is

\paper {
  #(define dump-extents #t)

  indent = 0\mm
  ragged-right = ##t
  force-assignment = #""
  line-width = #(- line-width (* mm  3.000000))
}
but looks better as 

\paper {
  #(define dump-extents #t)
  
  indent = 0\mm
  left-margin = 20\mm
  ragged-right = ##f
  force-assignment = #""
  line-width = 15\mm
}

But, of course, I read this in the CG:
If you are making an
example demonstrating special
\paper{} values, contact the
Documentation Editor. 

So, I'll leave it at that. If someone wants to make
these changes, I think the cheatsheet will look 
nicer. I can patch a change for the first one, but
I don't know how to touch the auto-generated \paper
code for the second one.

HTH
- Mark