[feature request] French/sostenuto slur/tie

[Malte Milatz]

Feature request: Add _direct_ support for a one-note slur/tie looking
approximately like this:

 |
 |         |
 |         |
O          |
 `--´     O
           `--´

(The drawing shows two notes, both having such a slur.)

Syntax proposal: c'() (this avoids any discussion about how it should be
called in proper English.)