[Feature] Accessibility

[anderoonies]

Summary

Other JS libraries like Pixi, Phaser, and Kontra handle accessibility in a way I think we could easily adopt.
Elements (subclasses of Thing) create "hidden" DOM elements (not actually hidden, in order to work with screen readers link). Tab interaction navigates between these, and the elements respond to focus events by becoming focused, which gives a border style. This can all be handled in thing.js with additional stroke.

Tab can be bound within the library to switch into this mode, and mouse movement can switch out of it. Elements can have default alt text that screen readers can read, and additional have alt text set via a method in Thing.

We'd need to make decisions about what should count as which interaction—mouseDown, mouseClick, mouseUp, etc. We have click, focus, blur, etc. we can use.

I'll make a proof of concept in the next few days.

[anderoonies]

This is partially addressed in #65