Support n-ary relation (trigger-less) links

[myedibleenso]

This may be related to #1.

Current Behavior

If a Link lacks a trigger, it is binarized:

• https://github.com/CreativeCodingLab/TextAnnotationGraphs/blob/697642dbec1daa7b9b6ab219c66ff8af335df50e/src/js/components/link.js#L1021-L1138

Expected behavior

n-ary trigger-less Links should be supported. We can mimic the approach followed to draw events (a Link with a trigger):

• https://github.com/CreativeCodingLab/TextAnnotationGraphs/blob/697642dbec1daa7b9b6ab219c66ff8af335df50e/src/js/components/link.js#L711