Feature: enable stacking events

[pdpino]

Fixes #233

(See demos in docs/overlaps.md)

[pdpino]

@hoangnm have you been able to check this PR?

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Sorry for the large PR, I'll add a small code walkthrough in case is useful 🙂:

• Moving stuff to box.js and types.js is refactoring (i.e. minor code changes or not at all)
• Moving stuff to position.js is mainly refactoring, plus support a stackOffset in left and width
• resolveEventOverlaps() function in overlap.js handles lane/stack/ignored cases:
  • the overall algorithm remains very similar (functions resolveEventOverlaps, addOverlappedToArray)
  • buildFromOverlappedEvents() handles different overlapped cases
  • The Classes are helpers to remember state throughout the process

[hoangnm]

This is really awesome, great solution and clean code, @pdpino .

[rdewolff]

Wouhou, amazing! Thanks for your hard work!