Add click() to Clickable.
Added a button that uses images for up/down state.

I'm not super thrilled with this because it can't extend Button and it's a
little annoying to have two different kinds of buttons in your code for code
that wants to operate indiscriminately on button like things.

Clickable doesn't really help because you want something that has Element's API
(setVisible, setEnabled) and Button's API (click, onClick, etc.).

It's also annoying to duplicate a non-trivial amount of code from Button. If I
had traits with self types, I could do all this properly. Or union types. Or
pink unicorns. Pink unicorns solve all problems.