`ResizeObserver` for more automatic `cy.resize()`s

[maxkfranz]

We already catch direct attribute changes on the container (e.g. el.classList.add()) for resizes, but using the proposed ResizeObserver would catch more cases and would avoid some false positives.

Once the spec is finalised, we can use it with a check for the API's existence. We can fall back on the existing behaviour if ResizeObserver is unavailable.

[TrySound]

https://github.com/que-etc/resize-observer-polyfill

[maxkfranz]

Thanks. That would allow for an easier transition when it's supported natively in the browsers. However, it seems like the ResizeObserver API is just a draft at this point and could change in future. We're already doing what the resize-observer-polyfill does under the hood with MutationObservers etc. So it probably doesn't make sense to take action on this now, but the polyfill could be useful when the ResizeObserver API is finalised.

[maxkfranz]

More browsers support this now or will support it soon: https://caniuse.com/#search=resizeobserver

Let's put this in the 3.9.0 milestone.

[maxkfranz]

I'm not getting errors with eslint when I run it locally:

$ node --version
v10.16.0
$ npm --version
6.9.0
$ npm run lint

> cytoscape@3.9.0-unstable lint /Users/max/Documents/workspace/cytoscape.js
> eslint src benchmark

$

[maxkfranz]

Though I can't reproduce the linting error, I have added comments to disable the eslint warnings in c18937b.