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