DefaultEventBus: avoid publishing events nobody is interested in

The AWT EventQueue is really slow: EventQueue.invokeAndWait() can take
up to 30x as long as creating a new thread and starting it on the
provided Runnable.

If there is not even a subscriber to the event we are about to send,
let's just not bother.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>