Invoked when the target {@link Clipboard} of the listener
has changed its available {@link DataFlavor}s.
Some notifications may be redundant — they are not
caused by a change of the set of DataFlavors available
on the clipboard.
For example, if the clipboard subsystem supposes that
the system clipboard's contents has been changed but it
can't ascertain whether its DataFlavors have been changed
because of some exceptional condition when accessing the
clipboard, the notification is sent to ensure from omitting
a significant notification. Ordinarily, those redundant
notifications should be occasional.
