Support for paste for the web backend.

[Emm]

Specific Demand

There is currently an onpaste handler. Unfortunately, the event itself appears to have no payload. It would be great to get a reference to the underlying DataTransfer or to a wrapper.

[ealmloff]

A platform agnostic wrapper around onpaste would be great. We could reuse FileEngine for the file section of the event and store a list of enums for the items in the paste event.

#1402 exposes the raw web-sys type for all events with a trait.