Motivating use case: Feynman diagrams

Graphite should support creating the Feynman diagrams. Users should be able to draw the vector paths and assign the particle types and labels. The node graph should allow for diagrams to be generated mostly automatically and then manually refined where necessary.

- [x] #2221
- [x] #696
- [x] #1832
- [ ] UI for assigning attributes from inside the path tool - ideally a dropdown with the particle type per segment
- [ ] Generating graphics probably requires some custom nodes along with #883
- [ ] Automatically place labels to attempt to avoid intersections



Start by reviewing the existing path tool and node graph, along with the completed work in #2221, #696, and #1832 and the related custom-node work in #883. The remaining scope includes assigning particle attributes per path segment, generating diagrams with custom nodes, and placing labels to avoid intersections. Done means users can create, automatically generate, and manually refine Feynman diagrams with labeled particle paths.

Written by the indexing model from the issue text.