Make arguments like Context.GrobName accessible as symbol lists

input/regression/tuplet-no-stems.ly

 \header{
   texidoc = "Removing Stem_engraver doesn't cause crashes."
 }
 
 
-\version "2.17.5"
+\version "2.17.6"
 
 \layout{
  \context{
    \TabStaff
-   \override TimeSignature stencil = ##f
+   \override TimeSignature.stencil = ##f
  }
  \context{
    \TabVoice
    \remove "Beam_engraver"
    \remove "Stem_engraver"
-   \override TupletBracket number-visibility = ##f
+   \override TupletBracket.number-visibility = ##f
  }
 }
 
 partition = {
     \times 2/3 { f8 g a }
 }
 
 
 \new TabStaff {
     \partition
 }