Add dead notes

[132ikl]

There should be a way to denote a note as a dead note, so that it appears as such:

  4/4
 │─5──────────────────│
 │─5──────────────────│
 │─2─────x──2──2──2───│
 │─2──2──x──2──2──2───│
 │─2──0─────0──0──0───│
 │─0──────────────────│

(Original tab)