Cautionary accidentals

[Garrett Fitzgerald]

>From Regression tests,  LilyPond 2.11.35

Test Description:
Cautionary accidentals are indicated using either parentheses
(default) or smaller accidentals.

Test display:
All accidentals are parenthesized.

Snippet:
    c''4
    cis''?4
    \once \override Staff.AccidentalCautionary #'font-size = #-2
    cis''?4
    \once \override Staff.AccidentalCautionary #'parenthesized = ##t
    cis''?4