Async canvas component / IE11 compatibility

[Marc-Andre-Rivet]

Followup to https://github.com/plotly/dash/pull/973

Now related to https://github.com/plotly/dash/pull/1006

[alexcjohnson]

LGTM! @emmanuelle I'll let you give final approval, just in case there's anything unusual - derived props or environment requirements... or if you have comments on any of the build changes here.

[alexcjohnson]

just a few new comments https://github.com/plotly/dash-canvas/pull/29#discussion_r343978751 https://github.com/plotly/dash-canvas/pull/29#discussion_r343964724 and there are still \t in several src files https://github.com/plotly/dash-canvas/pull/29#discussion_r342829994 - but these are all pretty minor concerns, overall looks great. Would like to have @emmanuelle take a look too though.

[Marc-Andre-Rivet]

Here's the docs with the candidate version: https://github.com/plotly/dash-docs/pull/699

Percy diffs were all related to (1) version change, (2) new arrow button

[emmanuelle]

Thank you @Marc-Andre-Rivet it looks good to me! :dancer: