add tied notes

[aqandrew]

• Whether a note is tied is boolean at the note level, not the duration level.
• A tied note can be rendered with parentheses: (0)
• Pressing L at a duration with no preceding notes on that string does nothing.
• If there are preceding notes,
  • If there are empty durations between the selected duration and the preceding note on that string,
    • The empty durations are filled to fill the bar(s), using the longest possible duration lengths first.

Before pressing L:

After pressing L: