Requests the contents of the clipboard as text.
When the text is later received, it will be converted to UTF-8 if necessary, and
callback will be called.
text parameter to
callback will contain the resulting text if the request succeeded, or
null if it failed. This could happen for various reasons, in particular if the clipboard was empty or if the
contents of the clipboard could not be converted into text form.
a function to call when the text is received, or the retrieval fails. (It will always be called one way or the other.)
user data to pass to