Fix duration of tied notes

[agourlay]

Currently we look only at the immediate next note to check if it is tied.

Actually we should check at all next beats in a current measure and a the next measures.

The goal is to detect precisely a chain of tied notes on a given string to compute correctly the note duration.

[agourlay]

Applies also to the "let ring" effect.

[agourlay]

Partially fixed with de958c2

Still need to handle longer chains