Added styling and clicking to buttons

Hi all,

The goal was to get Button to use `style_with` and `style_init` (see #711) which was a daunting task since that meant changing the SWT side of how button works (see [this comment](https://github.com/shoes/shoes4/pull/768#issuecomment-46875574)).

`button` `radio` `check` and `link` are all very related, especially in the backend. So in order to get this to work I had to work on all 4 at the same time (thankfully, I didn't have to do too much with link).

The good news is it works! And it passes specs (well on the surface, I'll get into that in the inline comments). It also resolves #748.

The bad news is, it's A LOT to change in one gulp, and even though it passes the standing specs, I still need to write the specs that *weren't* failing.

Unfortunately I won't be able to write the inline comments for another couple hours or so. So if you're eager feel free to dig in and play around (or at the very least write some clicky apps). Otherwise I'll give more details soon enough. 

:confetti_ball: 
I split accessor methods into reading and writing so that a custom method for either can be defined by the element (`state=` and `click` are prominent examples in this PR) and still let style handle read and write otherwise.