Make arguments like Context.GrobName accessible as symbol lists

input/regression/fret-diagrams-landscape.ly

-\version "2.16.0"
+\version "2.17.6"
 
 \header {
   texidoc="
 Fret diagrams can be presented in landscape mode.
 
 "
 }
 
 \layout { ragged-right = ##t }
 
 <<
   \chords {
     c1 |
     c1 |
     c1 |
     c1
   }
 
   \new Voice {
     \textLengthOn
-    \override TextScript #'fret-diagram-details
-       #'orientation = #'landscape
+    \override TextScript.fret-diagram-details.orientation = #'landscape
 
      %% C major for guitar, regular style
     c'1 ^\markup {
             \fret-diagram #"6-x;5-3-3;4-2-2;3-o;2-1;1-o;"}
 
      %% C major for guitar with barre on fret 3, regular style
-    \once \override TextScript #'fret-diagram-details·
-        #'label-dir = #LEFT
-    \once \override TextScript #'fret-diagram-details·
-        #'barre-type = #'straight
+    \once \override TextScript.fret-diagram-details.label-dir = #LEFT
+    \once \override TextScript.fret-diagram-details.barre-type = #'straight
     c' ^\markup {
             \fret-diagram·
                #"f:1;s:1.2;6-x;c:5-1-3;5-3-1;4-5-2;3-5-3;2-5-4;1-3-1;"}
 
      %% C major for guitar with two barres, regular style
 
     c' ^\markup {
             \fret-diagram·
               #"f:2;h:5;6-x;c:5-1-3;5-3-1;c:4-2-5;4-5-4;3-5-4;2-5-4;1-3-1;"}
 ···
      %% C major for guitar, with capo·
     c' ^\markup {
             \fret-diagram-verbose #'((mute 6)
                                      (capo 3)
                                      (place-fret 4 5 1)
                                      (place-fret 3 5 2)
                                      (place-fret 2 5 3))}
 
  }
 >>