Clipboard API: Proposal for asynchronous API

I just sent a proposal to the WICG for an asynchronous Clipboard API:
https://discourse.wicg.io/t/proposal-modern-asynchronous-clipboard-api/1513

(yes, I know. Not the first time something like this has been proposed
<https://lists.w3.org/Archives/Public/public-webapps/2015JulSep/0235.html>)

This is a follow-up to an earlier thread
Clipboard API: remove dangerous formats from mandatory data types
<https://lists.w3.org/Archives/Public/public-webapps/2015AprJun/0819.html>
where Chrome is unable to safely add clipboard support for a number of
image formats.

An async API would allow these image formats to be supported (because the
sanitization could happen without blocking the main thread).