Support scene accelerators

[cebaa]

A shortcut for this:

Observable.generate(emitter -> {
         scene.getAccelerators().put(new KeyCharacterCombination("c"),
            () -> emitter.onNext(true));
      });

[thomasnield]

Can you put in a PR, thanks!

[rcd27]

What kind of shortcut do you want to see as result?
val keyCharObservable = eventsOf(scene, EventType<KeyCharacterCombination, "c">)?

[cebaa]

@rcd27

Since the signature is:

public ObservableMap<KeyCombination,Runnable> getAccelerators()

so maybe something like this:

val keyCharObservable = withAccelerator(scene, new KeyCharacterCombination("c"))

[rcd27]

@thomasnield I think I'm able to do that