Differentiate between overlay clicks and target clicks

[percula]

This PR resolves #28

I added a callback onClickOverlay that is called when the overlay is clicked. Also, onClickTarget is now called only when the target is clicked.

My use case for this is that I have a FloatingActionButton that is the target. When the overlay is clicked, I just want to dismiss the tutorials. But when the target is clicked, I want to do the action in the FloatingActionButton's onPressed method.

[RafaelBarbosatec]

Thanks @percula ! Greate job!