⌂ Clef, key and time signature in square brackets

For critical editions, objects like clefs, key or time signatures sometimes need to be written in parentheses to indicate they are missing from the autograph. This can be achieved by simply wrapping the bracketify-stencil function around the object's stencil function. By using \once \override, the effect can be applied to just one clef/key/time signature; by using \override it applies to the whole piece.