Fix: Regression from #605 and also allow for [prev measure/system] in…

[worldwideweary]

… note-entry even when input state has no ChordRest

Resolves: #604 regression and enabling previous measure/previous system when input track doesn't contain a correspondent chordrest

That should do it. Unfortunately that means your "up-port" (is that the word?) would also need to be updated

Verification:

1. Traversal through grace notes is verified:

1.webm

2. Reason for issue is verified:

2.webm

3. Update: allow previous measure and previous system (switches to voice one, but initially wasn't working at all when in that input state of not having any existing chordrest in 3.6.2 etc):

3.webm