Choose single or double bar line at segno/repeat

The new context property underlyingRepeatType (default ||) specifies the type of bar line to use at points of repetition and departure when no bar line (including a measure bar line) would otherwise appear.

Even if a repeat bar line will appear, the value of underlyingRepeatType is appended to the name of the repeat bar line as an annotation, provided that the result names a defined bar line. This enables using distinct bar lines at line breaks.

The default value of segnoType has been changed back to S now that Repeat_acknowledge_engraver can create a double bar line conditionally.

This deserves an entry in the Changes file and coverage in the Notation Reference, but I would like to create a ticket for that and batch it with some of the changes that are queued up behind it.