Annotating and visualizing precedence

[hajicj]

It should be possible to annotate and visualize not only the symbol attachment graph, but also the precedence graph of notes and rests (however it is defined).

This essentially means having edges of different types in the object graph. There are two ways to implement this in the data model:

• Hold in one graph and have different edge labels: use edge labels to decide how to render edge; when synchronizing graph to CropObjects, use edge labels to decide how to modify the CropObject (inlinks/outlinks vs. data)

• Hold in separate graphs.

Keeping all the edges in one graph has the disadvantage that it will be difficult to have two edges with different labels between a pair of objects. We can record label lists, but that breaks the 1:1 relationship between edge in graph and EdgeView.

[hajicj]

Steps towards solution 1:

• [x] When adding a precedence edge, the label param on model.ensure_add_edge() will be non-default (label='Precedence')
• [x] The EdgeView has to set its color (resp. other visual properties) according to the edge label. This should be handled in EdgeView._generate_edge_view_color(), which needs to get a label param.
• [ ] In order to implement synchronization: the data model for precedence edges also needs to be specified for the CropObject itself. Since precedence inlinks and outlinks are lists, this may require adding list functionality to CropObject.data entries.

[hajicj]

Decision: so far, we will stick with solution 1. It requires the least modification and (almost) by definition, attachment edges and precedence edges will never lead between the same pair of symbols: it is actually right now forbidden to have Attachment among noteheads/rests.