DialogService:: add input box with prompt text

[AlmasB]

The API should be consistent with DialogService. Something along the lines of:

public abstract void showInputBox(String message, String promptText, Predicate<String> filter, Consumer<String> resultCallback);

and also

public abstract void showInputBox(String message, String promptText, Consumer<String> resultCallback);

The implementation is in FXGLDialogService (com.almasb.fxgl.app.services.FXGLDialogService). At first, this seems to be a reasonably easy issue as we might be able to use an existing implementation and just set promptText.