Timeline arrow

[seanlowe]

v2.4.0 (continued)

Implement Issues:

• [Feature - Horizontal] Implement Timeline-arrow as a way of connecting events #44

Changes:

• timeline-arrow integration:
  • added a new event property pointsTo to allow users to specify a target event to link to
  • wrote new function makeArrowsArray to handle finding links and creating the array of arrows to attach to the timeline
  • updated Insert commands to add the new property
  • updated the docs to reflect the new functionality

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resolves #44

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part of release / milestone 2.4.0

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still not updating the versioning due to at least 1 more issue planned for 2.4.0