Add Safari/Webkit support for clipboard (aync api)

The current FAQ mentions mostly Chrome for supporting writing/reading from the clipboard via the newer async clipboard API, which doesn't require bringing up the Guacamole menu. Webkit has seemingly adopted this API as of [June 2020|https://webkit.org/blog/10855/async-clipboard-api/.]

The FAQ also mentions some browsers implement restrictions, and there is a section in the webkit announcement about clicks required, etc. It would be nice to see if this could be revisited and see the behavior in Chrome today can be matched.