clipboard images with html5 client

[Antoine Martin]

We want to transfer the pixel data as image/png and set it as best we can using setData or newer methods (#1844).

See ​THE DEFINITIVE GUIDE TO COPYING AND PASTING IN JAVASCRIPT

The difficulty here is that because the html5 clibpoard is greedy, we need to choose png as soon as we get the clipboard event so we can send the binary data with the token.