Make Button class concrete

[crussell52]

Currently, the Button class is abstract but inherits from a concrete class (Trigger) and provides no additional abstract signatures. This prevents Button from being directly instantiated, so it loses the BooleanSupplier benefits that have been added to Trigger.

For example, the following is not possible:

new Button(() -> myButtonLikeInput.isPressed()).whenPressed(myCommand);

[kasemir]

Just noticed the same yesterday when we wanted to use one of the triggers as a 'button'.
Can do this:

new Trigger(() -> joystick.getTriggerAxis(Hand.kRight) > .5)

.. but allowing actual Button support would be nice:

new Button(() -> joystick.getTriggerAxis(Hand.kRight) > .5)

Thanks for providing a pull request!