consider adding capability to copy-paste image from clipboard

[ltalirz]

See https://stackoverflow.com/a/50427897/1069467

Until this is supported by the webplotdigitizer as well, however, people will still have to save the screenshot in a file, making this not very useful.

[ltalirz]

It actually is supported by the webplot digitizer

[ltalirz]

I've added the JS part to set the content of the file input from copy-paste in
9e44622

However, for some reason, this does not seem to trigger an update of the object on the python side.
Probably one has to figure out how to interact with the PanelJS library