async clipboard api

[Antoine Martin]

See ​Unblocking Clipboard Access: It's a replacement for execCommand-based copy & paste that has a well-defined permissions model and doesn't block the page.

MDN: ​Navigator clipboard.

Original html5 clipboard tickets: #842, #1461

[Antoine Martin]

See also related work in #812

[Antoine Martin]

For images, see #2312

[Antoine Martin]

Blocker for #2312

[Antoine Martin]

Updates:

• r22834 new async api calls
• r22837 answer clipboard requests
• r22838 workaround PRIMARY + CLIPBOARD overwriting values
• r22839 don't bother synchronizing PRIMARY if we have the new async API
• r22840 stricter test for navigator.clipboard API availability

Important: just found information on ​clipboardchange event in ​w3c: Async Clipboard API. This is not supported in any browser yet.
This means we would not necessarily have to claim the clipboard every time we get a token, we could rely on those events to notify the server.
For chrome: ​onClipboardDataChanged.

Maybe we should query the ​Permissions API rather than just checking if the method exists?
See ​Asynchronous Clipboard API Sample

Tested:

• Chrome 75.0.3770.51 (Official Build) beta (64-bit) Linux: OK (connecting to localhost using websocket)
• Chrome 74 MS Windows: OK via https, otherwise legacy mode
• Firefox 67 Linux: legacy mode
• Firefox 67 MS Windows: legacy mode

The Firefox results are surprising since they claim to support the async API since V63, but this matches the results from ​this test.
The spec says: Firefox only supports reading the clipboard in browser extensions, using the "clipboardRead" extension permission.

Then there are also issues with "secure contexts": works for localhost, otherwise https is required, etc..