Create way of adding and deleting precedence edges

[hajicj]

Once the data model and visualization for precedence edges is implemented in #161, we need to also enable adding the edges.

Keyboard shortcut: n

Operation: from the current selection, add precedence edges between horizontally adjacent (simultaneity-equivalent) noteheads and rests, sorted left to right.

In order for the operation to produce a correct precedence graph:

• Only ever have one voice selected (but chords are OK, as long as the noteheads share a stem)
• Only select objects from one staff