public void request_targets (ClipboardTargetsReceivedFunc callback)
Requests the contents of the clipboard as list of supported targets.
When the list is later received,
callback will be called.
targets parameter to
callback will contain the resulting targets if the request succeeded, or
null if it failed.
a function to call when the targets are received, or the retrieval fails. (It will always be called one way or the other.)
user data to pass to